Chapter D: D, an apparatus for holding a small tube, that the spectrum given by (1)

its contents may be compared with that from any other object on the stage.

E, a screw, opening and shutting a slit to admit the quantity of light required to form the second spectrum. Light entering the aperture near E strikes against the right-angled prism which we have mentioned as being placed inside the apparatus, and is reflected up through the slit belonging to the compound prism. If any incandescent object is placed in a suitable position with reference to the aperture, its spectrum will be obtained, and will be seen on looking through it.

F shows the position of the field lens of the eye-piece.

G is a tube made to fit the microscope to which the instrument is applied. To use this instrument, insert G like an eye-piece in the microscope tube. Screw on to the microscope the object-glass required, and place the object whose spectrum is to be viewed on the stage. Illuminate with stage mirror if transparent, with mirror and lieberkühn and dark well if opaque, or by side reflector, bull’s-eye, &c. Remove A, and open the slit by means of the milled head, H, at right angles to D D. When the slit is sufficiently open the rest of the apparatus acts like an ordinary eye-piece, and any object can be focussed in the usual way. Having focussed the object, replace A, and gradually close the slit till a good spectrum is obtained. The spectrum will be much improved by throwing the object a little out of focus.

Every part of the spectrum differs a little from adjacent parts in refrangibility, and delicate bands or lines can only be brought out by accurately focussing their own parts of the spectrum. This can be done by the milled head, B. Disappointment will occur in any attempt at delicate investigation if this direction is not _carefully attended to_. When the spectra of very small objects are to be viewed, powers of from ½ in. to 1/20th, or higher, may be employed. Blood, madder, aniline dyes, permanganate of potash solution, are convenient substances to begin experiments with. Solutions that are too strong are apt to give dark clouds instead of delicate absorption bands. Small cells or tubes should be used to hold fluids for examination.

Mr. Browning has still further improved the micro-spectroscope by the ingenious arrangement for measuring the positions of the lines, which is represented in Fig. 225, and the construction and the use of which he thus described in a paper read before the Microscopical Society:

FIG. 225.—_Section of Micro-Spectroscope with Micrometer._
]

Attached to the side is a small tube, A A. At the outer part of this tube is a blackened glass plate, with a fine clear white pointer in the centre of the tube. The lens, C, which is focussed by sliding the milled ring, M, produces an image of the bright pointer in the field of view by reflection from the surface of the prism nearest the eye. On turning the micrometer, M, the slide which holds the glass plate is made to travel in grooves, and the fine pointer is made to traverse the whole length of the spectrum.

It might at first sight appear as if any ordinary spider’s web or parallel wire micrometer might be used instead of this contrivance. But on closer attention it will be seen that as the spectrum will not permit of magnification by the use of lenses, the line of such an ordinary micrometer could not be brought to focus and rendered visible. The bright pointer of the new arrangement possesses this great advantage—that it does not illuminate the whole field of view.

If a dark wire were used, the bright diffused light would almost obscure the faint light of the spectra, and entirely prevent the possibility of seeing, let alone measuring, the position of lines or bands in the most refrangible part of the spectrum.

To produce good effects with this apparatus the upper surface of the compound prism, P, must make an angle of exactly 45° with the sides of the tube. Under these circumstances the limits of correction for the path of the rays in their passage through the dispersing prisms are very limited and must be strictly observed. The usual method of correcting by the outer surface is inadmissible. For the sake of simplicity, some of the work of the lower part of the micro-spectroscope is omitted in the engraving. As to the method of using this contrivance: With the apparatus just described, measure the position of the principal Fraunhofer’s lines in the solar spectrum. Let this be done _carefully_, in _bright_ daylight. A little time given to this measurement will not be thrown away, as it will not require to be done again. Note down the numbers corresponding to the position of the lines, and draw a spectrum from a scale of equal parts. About 3 in. will be found long enough for this spectrum; but it may be made as much longer as is thought desirable, as the measurements will not depend in any way on the distance of these lines apart, but only on the micrometric numbers attached to them. Let this scale be done on cardboard and preserved for reference. Now measure the position of the dark bands in any absorption spectra, taking care for this purpose to use lamplight, as daylight will give, of course, the Fraunhofer lines, which will tend to confuse your spectrum. If the few lines occurring in most absorption spectra be now drawn to the same scale as the solar spectrum, on placing the scales side by side, a glance will show the exact position of the bands in the spectrum relatively to the Fraunhofer lines, which thus treated form a natural and unchangeable scale (see diagram, Fig. 226). But for purposes of comparison it will be found sufficient to compare the two lists of numbers representing the micrometric measures, simply exchanging copies of the scale of Fraunhofer lines, or the numbers representing them will enable observers at a distance from each other to compare their results, or even to work simultaneously on the same subject.

FIG. 226.
]

A simpler form of the micro-spectroscope is also made by Mr. Browning at a very modest price, and if the reader possesses a microscope, and desires to examine these interesting subjects for himself, he will do well to procure this instrument, instead of that represented in Fig. 220, as it will also answer better for other purposes. A section of the instrument is shown in Fig. 227. When used with the microscope it is slipped into the place of the eye-piece. There is an adjustable slit, a reflecting prism, by which two different spectra may be examined at once, and a train of five prisms for dispersing the rays. It can be used equally well for seeing the bright lines of metals and the Fraunhofer lines, and for viewing any two spectra simultaneously. These direct-vision spectroscopes are better adapted for general use by those who have not several different instruments, than such forms as that shown in Fig. 229, for in the direct-vision instruments the whole extent of the spectrum is visible at one view, which is by no means the case with the larger instruments.

FIG. 227.—_Section of Micro-Spectroscope._
]

_CELESTIAL CHEMISTRY AND PHYSICS._

We now approach that portion of our subject in which its interest culminates, for however remarkable may be some of the above-named results of this searching optical analysis, they are surpassed by those which have been obtained in the field upon which we are about to enter. The cause of the dark lines which Fraunhofer observed in the light of the sun and of certain stars remained unexplained, he only establishing the fact that they must be due to some absorptive power existing in the sun and stars themselves, and not to anything in our atmosphere. It was reserved for Professor Kirchhoff, of the University of Heidelberg, to show the full significance of the dark lines. Fraunhofer had, on his first observation of the lines, noticed that the D lines were coincident with the bright lines in the spectrum of sodium. This interesting fact may be readily observed with any spectroscope which permits of the two spectra being simultaneously viewed. The bright line (or lines if the spectroscope be powerful) of the metal is seen as a prolongation of the dark D solar line. Even with an instrument like that shown in Fig. 220 the coincidence may be noticed. Let the observer receive into the instrument the rays in diffused daylight only, when he will still see the principal Fraunhofer lines distinctly, and let him note the exact position of the D line, while he brings in front of the slit the flame of a spirit-lamp charged with a little salt. He will then see the bright yellow line replacing the dark D line, and by alternately removing and putting back the lamp he will be soon convinced of the perfectly identical position of the lines.

This fact remained without explanation from 1814 to 1859, when Kirchhoff accidentally found, to his surprise, that the dark D line could be produced artificially. He says: “In order to test in the most direct manner possible the frequently asserted fact of the coincidence of the sodium lines with the D lines, I obtained a tolerably bright solar spectrum, and brought a flame coloured by sodium vapour in front of the slit. I then saw the dark lines D, change into bright ones. The flame of a Bunsen’s lamp threw the bright sodium lines upon the solar spectrum with unexpected brilliancy. In order to find out the extent to which the intensity of the solar spectrum could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame, and, to my astonishment, I saw that the _dark lines_, D, _appeared with an extraordinary_ degree of clearness. I then exchanged the sunlight for the Drummond’s or oxy-hydrogen lime-light, which, like that of all incandescent solid or liquid bodies, gives a spectrum containing no dark lines. When this light was allowed to fall through a suitable flame, coloured by common salt, _dark_ lines were seen in the spectrum in the position of the sodium lines. The same phenomenon was observed if, instead of the incandescent lime, a platinum wire was used, which, being heated in the flame, was brought to a temperature near its melting point, by passing an electric current through it. The phenomenon in question is easily explained, upon the supposition that the sodium flame absorbs rays of the same degree of refrangibility as those it emits, whilst it is perfectly transparent for all other rays.” (Quoted in Roscoe’s Lectures on “Spectrum Analysis.”) When the light of ignited lime was similarly made to pass through flames containing the incandescent vapours of potassium, barium, strontium, &c., the bright lines which these substances would have produced had the lime-light not been present were found to be in every case changed into dark lines, occupying the very same positions in the spectrum. In such experiments the flames containing the metals in the vapourized state do all the time really give off those rays which are peculiar to each substance; but when a more intense illumination—such as the lime-light, the electric arc, or direct sunlight—passes through them, the rays of the spectrum produced by the intense light overpower those given off by the relatively feebly coloured flames, and hence the portions of the spectrum which are occupied by these, appear black. But as the intense light would give a perfectly continuous spectrum if the incandescent metallic vapour allowed the rays corresponding to its lines to pass through it, the inference is obvious that each vapour absorbs those particular rays which it has itself the power of emitting, but allows all others to pass freely through it. Besides the experimental proofs of this fact which have been already adduced, many others might be named. The flame of a spirit-lamp with a salted wick appears opaque and smoky when we look through it at a large flame of burning hydrogen, also coloured by sodium; for the rays emitted by the latter do not penetrate the former, which, in consequence of its feebler light, appears dark by comparison. Again, if an exhausted tube containing metallic sodium be heated so as to convert the sodium into vapour, the tube viewed by the light of a sodium flame appear to contain a black smoke, and the light from the flame will no more pass through it than through a solid object; yet the tube appears perfectly transparent when viewed by ordinary light, and the light from a lithium or other coloured flame would also pass freely. Kirchhoff was led by purely theoretical reasoning to conclude that all luminous bodies have precisely the same power of absorbing certain rays of light as they have of emitting them at the same temperature, and he thus brought luminous rays under the same general law which had previously been established for radiant heat by Prevost, Dessains, Balfour Stewart, and others. Here, then, a law was arrived at, and, abundantly confirmed by direct experiment as regards the more volatile metals, it was ready to supply the most satisfactory explanation of the coincidences which were everywhere discovered to exist between the Fraunhofer lines and those which belong to terrestrial substances. For Kirchhoff also found, when mapping the very numerous lines seen in the spark spectrum of iron, that for each of the 90 bright lines of iron which he then observed, there was a dark line in the solar spectrum exactly corresponding in position. The number of observed bright lines in the iron spectrum has been since extended to 460, and yet each is found to have its exact counterpart in a solar dark line.

So many coincidences as these made it certain that these dark lines and the bright lines of iron must have a common cause, for the chances against the supposition that the agreement was merely accidental are enormous. Kirchhoff actually calculated, by the theory of probabilities, the odds against the supposition. He found it represented by 1,000,000,000,000,000,000 to 1. The result arrived at in the case of sodium at once suggested the explanation that these lines were produced by an absorptive effect of the vapour of iron. Now, the existence of such a vapour in our atmosphere could not be admitted, while the temperature of the sun was known to be exceedingly high, far higher, indeed, than any temperature we can produce by electricity, or any other means. Hence, Kirchhoff concluded that his observations proved the presence of the vapour of iron in the sun’s atmosphere with as much certainty as if the iron had been actually submitted to chemical tests. By the same reasoning, Kirchhoff also demonstrated the existence in the solar atmosphere of calcium, chromium, magnesium, nickel, barium, copper, and zinc. To these, other observers have added strontium, cadmium, cobalt, manganese, lead, potassium, aluminium, titanium, uranium, and hydrogen. It has also been demonstrated that a considerable number of the Fraunhofer lines are due to absorption in our atmosphere by its gases and aqueous vapour. This demonstration of the existence of iron and nickel in the sun is an interesting pendent to the known composition of many meteorites which reach us from interplanetary space.

Kirchhoff was led to believe that the central part of the sun is formed of an incandescent solid or liquid, giving out rays of all refrangibility, just as white-hot carbon does; that round this there is an immense atmosphere, in which sodium, iron, aluminium, &c., exist in the state of gas, where they have the power of absorbing certain rays; that the solar atmosphere extends far beyond the sun, and forms the corona; and that the dark sunspots, which astronomers have supposed to be cavities, are a kind of cloud, floating in the vaporous atmosphere.

During total eclipses of the sun, certain red-coloured prominences have been noticed projecting from the sun’s limb, and visible only when the glare of its disc is entirely intercepted by the moon. Fig. 228 represents a total eclipse, and will give a rude notion of the appearance of the red prominences seen against the fainter light of the _corona_, which extends to a considerable distance beyond the sun’s disc. Now, two distinguished men of science simultaneously and independently made the discovery of a mode of seeing these red prominences, even when the sun was unobscured. M. Janssen was observing a total eclipse of the sun in India, and the examination by the spectroscope of the light emitted from the red prominences showed him that they were due to immense columns of incandescent hydrogen, for he recognised the red line and blue lines which belong to the spectrum of this gas (see No. 12, Plate XVII.). Mr. Norman Lockyer at the same time also succeeded in viewing the solar prominences in London without an eclipse. He found a red line perfectly coinciding in position with Fraunhofer’s C line and that of hydrogen, another nearly coinciding with F, and a third yellow line near D. Soon after this, Dr. Huggins discovered a mode of observing the shape of the red prominences at any time, by using a powerful train of prisms and a wide slit, so that the changes in the forms of the red flames can be followed. Now, since the red prominences give off only a few rays of particular refrangibility, it is not difficult to understand that the light of the sun might be, as it were, so diluted by stretching out the spectrum, by means of a train of many prisms, that almost only the red rays, C, should enter the telescope, and occupy the field with sufficient intensity to overpower all others, and produce an image of the object from which they originated. The nature of this action may be illustrated thus: If we hold vertically a prism, and look through it at a candle-flame, we may perceive a lengthened-out image of the flame, showing the succession of prismatic colours, and formed, as it were, of a red image of the flame close to a yellow one, and so on, but presenting no defined form. If, still viewing this spectrum, we introduce into the flame on a platinum wire a piece of common salt, we shall perceive a well-defined yellow image of the candle start out, because the rays which are emitted by the incandescent sodium, being all of one refrangibility, the prism simply refracts without dispersing them. The dispersion which weakens the light of the continuous spectrum by lengthening it out, does not sensibly detract from the brilliancy of the bright lines, as their breadth is scarcely increased—they are refracted but not dispersed. Hence, when a sufficient number of prisms is employed, the bright lines of the solar _chromosphere_ may be seen in full sunshine, in spite of the greater intensity of the light emanating from the _photosphere_, which produces the continuous spectrum. The bright C line is, of course, a virtual image of the slit produced by rays of that particular refrangibility; but by using a very high dispersive power, the slit may be opened so wide that the C rays form in the telescope a red image of the prominence from which they issue, since their light will predominate over that of any rays belonging to the continuous spectrum.

FIG. 228.—_Solar Eclipse, 1869._
]

In the hands of Mr. Norman Lockyer the science of the physical and chemical constitution of the sun has made rapid progress, and new facts are continually being observed, which serve to furnish more and more definite views. Mr. Lockyer considers that, extending to a great distance around the sun is an atmosphere of comparatively cooler hydrogen, or perhaps of some still lighter substance which is unknown to us. It is this which forms what is termed the _corona_, or circle of light which is seen surrounding the sun in a total eclipse. Immersed in this, and extending to a much smaller distance from the nucleus of the sun, is another envelope, termed the _chromosphere_, consisting of incandescent hydrogen and some glowing vapours of magnesium and calcium. The brightest part of this envelope, which lies nearest the sun, is that which gives off the red rays by which the prominences may be observed without an eclipse. These prominences have been shown to be tremendous outbursts of glowing hydrogen, belched up with sometimes an enormous velocity from below, since they have been observed to spring up 90,000 miles in a few minutes. Beneath the chromosphere, and nearer to the body of the sun, are enormous quantities of the vapours of the different elements—sodium, iron, &c.—to which the dark lines of the solar spectrum are due. This stratum Mr. Lockyer calls the _reversing layer_, because it reverses (turns to dark) the lines which would otherwise have appeared bright, just as Kirchhoff’s sodium vapour did in the experiment described on page 437. Beneath the reversing layer is the _photosphere_, from which emanates the light that is absorbed in part by the reversing layer, and which there is good reason to believe is either intensely heated solid or liquid matter.

In 1861 Dr. Huggins devoted himself, with an ardour which has since known no remission, to the extension of prismatic analysis to the other heavenly bodies. The difficulties of the investigations were great. There was first the small quantity of light which a star sends to the spectator; this was obviated by the use of a telescope of large aperture, which admitted and brought to a focus many more rays from the star, and therefore the brightness of the image was proportionately increased. Not so the size of the image: the case of the fixed stars for this always remains a mere point. It was, of course, necessary to drive the telescope by clockwork, so that the light of the star might be stationary on the field of the spectroscope. As the spectrum of the image of the star formed by the object-glass would be a mere line, without sufficient breadth for an observation of the dark or light lines by which it might be crossed, it is necessary to spread out the image so that the whole of the light may be drawn out into a very narrow line, having a length no greater than will produce a spectrum broad enough for the eye to distinguish the lines in it. This is accomplished by means of a cylindrical lens placed in the focus of the object-glass, and immediately in front of the slit. Covering one-half of the slit is a right-angled prism by which the light to be compared with that of the star is reflected into the slit. The light is usually that produced by taking electric sparks between wires of the metal in the manner already described. The dispersive power of the spectroscope was furnished by two prisms of very dense glass, and the spectrum was viewed through a telescope of short focal length. Dr. Huggins’s observations lead him to the conclusion that the planets Mars, Jupiter, and Saturn possess atmospheres, as does also the beautiful ring by which Saturn is surrounded; for he noticed in the spectrum of each different dark lines not belonging to the solar spectrum.

FIG. 229.—_The Planet Saturn._
]

Passing to the results obtained in the case of the fixed stars, we may remind the reader of the enormous distance of the bodies which are submitted to the new method of analysis. Sir John Herschel gives the following illustration of the remoteness of Sirius—supposed to be one of the nearest of the fixed stars: Take a globe, 2 ft. in diameter, to represent the sun, and at a distance of 215 ft. place a pea, to give the proportionate size and distance of the earth. If you wish to represent the distance of Sirius _on the same scale_, you must suppose something placed _forty thousand miles_ away from the little models of sun and earth. But not only do we know with certainty some of the substances contained in Sirius, but the star spectroscope has taught us a great deal about orbs so remote, that their distance is absolutely unmeasurable. About Aldebaran we know that there are hydrogen gas and vapours of magnesium, iron, calcium, sodium, and some four or five other elements. Generally the lines indicate the presence of hydrogen in these distant suns; but there is, at least, one remarkable exception in α _Orionis_, the spectrum of which yields no trace of the hydrogen lines, although it is evident that magnesium, sodium, calcium, &c., are present. The spectra of celestial bodies are of several kinds. Many of the stars have, like our sun, a continuous spectrum crossed by dark lines. Such is that of Sirius, No. 10, Plate XVII. Others have, however, both dark and bright lines, and some are marked by only three bright spaces. Of the spectra of the nebulæ some have three bright lines (see No. 11, Plate XVII.), and the bodies producing them are, therefore, to be considered as masses of incandescent gas, while some give continuous spectra. One of the bright lines in the spectra of the nebulæ coincides with one of the hydrogen lines, and another—the brightest of the three—with one of the brightest nitrogen lines; but the third does not agree with any with which it has as yet been compared. The inference from these appearances is that the nebulæ contain hydrogen and nitrogen, but the absence of the other lines of these substances has not been fully explained; although the observation of Dr. Huggins, that when the light of incandescent nitrogen and hydrogen is gradually obscured by interposing layers of neutral tinted glass, the lines corresponding with those in the nebular spectra are the last to disappear, seems to suggest a probable solution of the difficulty.

FIG. 230.—_Solar Prominences, No. 1._
]

There is another very interesting line of spectroscopic research in the power the prism gives us of estimating the velocity with which the distances of the stars from our system are increasing or diminishing. On closely examining the hydrogen lines of Sirius, and comparing them with the bright lines of hydrogen rendered incandescent by electric discharges in a Geissler tube, the spectrum of which his instrument enabled him to place side by side with that of the star, Mr. Huggins was surprised to find that the lines in the latter did not exactly coincide in position with those of the former, but appeared slightly nearer the red end of the spectrum. This indicated a longer wave-length, or increased period of vibration, according to the theory of light, which would be accounted for by a receding motion between Sirius and the earth, just as the crest of successive waves of the sea would overtake a boat going in the same direction at longer intervals of time than those at which they would pass a fixed point, while, if the boat were meeting the waves, these intervals would, on the other hand, be shorter. Hence if the position of the lines in the spectrum depends on the periods of vibration, that position will be shifted towards the red end when the luminous body is receding from the earth with a velocity comparable to that of light, and towards the violet end when the motion is one of approach. The change in refrangibility observed by Mr. Huggins corresponded with a receding velocity of 41·4 miles per second, and when from this was subtracted the known speed with which the earth’s motion round the sun was carrying us from the star at the time, the remainder expressed a motion of recession amounting to about twenty miles a second, which motion, there is reason to believe, is chiefly due to a proper movement of Sirius. These deductions from prismatic observations are of the highest value astronomically, since they will eventually enable the real motions of the stars to be determined, for ordinary observation could only show us that component of the motion which is at right angles to the visual ray, while this gives the component along the visual ray. In the same manner, it is inferred that Arcturus, a bright star in the constellation _Boötes_, is approaching us with a velocity of fifty-five miles per second.

FIG. 231.—_Solar Prominences, No. 2._
]

When the solar spots are examined with the spectroscope, the dark image of the slit produced by the hydrogen line, F, is observed to show a strange crookedness when it is formed by rays from different parts of the spot. This distortion is due to the same cause as the displacement of the stellar lines, namely, motions of approach or recession of the masses of glowing hydrogen. Mr. Norman Lockyer, to whom we are indebted for the most elaborate investigations of the solar surface, has calculated, from the position of the lines, the velocities with which masses of heated hydrogen are seen bursting upwards, and those which belong to the down-rushes of cooler gas. Velocities as great as 100 miles per second were, in this way, inferred to occur in some of the storms which agitate the solar surface. Two drawings of a solar storm, given by Mr. Lockyer, are shown in Figs. 230 and 231. These are representations of one of the so-called red prominences, the first giving its appearance at five minutes past eleven on the morning of March 14th, 1869, and the last showing the same _ten minutes afterwards_. The enormous velocity which these rapid changes imply will be understood when it is stated that this prominence was 27,000 miles high. “This will give you some idea,” says Mr. Lockyer, “of the indications which the spectroscope reveals to us, of the enormous forces at work in the sun, merely as representing the stars, for everything we have to say about the sun the prism tells us—and it was the first to tell us—we must assume to be said about the stars. I have little doubt that, as time rolls on, the spectroscope will become, in fact, almost the pocket companion of every one amongst us; and it is utterly impossible to foresee what depths of space will not in time be gauged and completely investigated by this new method of research.”

The light of comets has also been examined by the spectroscope, and many interesting results arrived at. Our limits do not, however, permit us to enter into a discussion of these interesting subjects.

Fig. 232 is a section of another of Mr. Browning’s popular instruments, which is named by him the “Amateur’s Star Spectroscope.” It exhibits very distinctly the different spectra of the various stars, nebulæ, comets, &c.

FIG. 232.—_Section of Amateur’s Star Spectroscope._
]

The reader who is desirous of learning more of this fascinating subject is referred to Dr. Roscoe’s elegant volume, entitled, “Lectures on Spectrum Analysis.” This work, which is embellished with handsome engravings and illustrated by coloured maps and spectra, gives a clear and full account of every department of the subject, and in the form of appendices, abstracts of the more important original papers are supplied, while a complete list is given of all the memoirs and publications relating to the spectroscope which have been published.

This brief account of the spectroscope and its revelations, which is all that our space permits us to give, will not fail to awaken new thoughts in the mind of a reader who has obtained even a glimpse of the nature of the subject, especially in relation to that branch of which we have last treated, for in every age and in every region the stars have attracted the gaze and excited the imagination of men. The belief in their influence over human affairs was profound, universal, and enduring; for it survived the dawn of rising science, being among the last shades of the long night of superstition which melted away in the morning of true knowledge. Even Francis Bacon, the father of the inductive philosophy, and old Sir Thomas Browne, the exposer of “Vulgar Errors,” believed in the influences of the stars; for while recognizing the impostures practised by its professors, they still regarded astrology as a science not altogether vain. It was reserved for the mighty genius of Newton to prove that in very truth there are invisible ties connecting our earth with those remote and brilliant bodies—ties more potent than ever astrology divined; for he showed that even the most distant orb is bound to its companions and to our planet by the same power that draws the projected stone to the ground. And now the spectroscope is revealing other lines of connection, and showing that not gravitation alone is the sympathetic bond which unites our globe to the celestial orbs, but that there exists the closer tie of a common constitution, for they are all made of the same matter, obeying the same physical and chemical laws which belong to it on the earth. We learn that hydrogen, and magnesium, and iron, and other familiar substances, exist in these inconceivably distant suns, and there exhibit the identical properties which characterize them here. We confirm, by the spectroscope, the fact partially revealed by other lines of research, that the stars which appear so fixed, are, in reality, careering through space, each with its proper motion. We learn also that the stars are the theatres of vast chemical and physical changes and transformations, the rapidity and extent of which we can hardly conceive. There is, for example, the case of that wonderful star in the constellation of the Crown, which, in 1866, suddenly blazed out, from a scarcely discernible telescopic star, to become one of the most conspicuous in the heavens, and the bright lines its beams produced in the spectroscope revealed the fact that this abrupt splendour was due to masses—who can imagine how vast?—of incandescent hydrogen. This brightness soon waned, and τ _Coronæ Borealis_ reverted once more to all but telescopic invisibility. The seeming fixity of the stars is an illusion of the same nature as that which prevents a casual observer from recognizing their apparent diurnal motion, and now we have also ample evidence that permanence of physical condition, even in the stars, is impossible. Everywhere in the universe there is motion and change; there is no pause, no rest, but a continual unfolding, an endless progression.

“Know the stars yonder,
The stars everlasting,
Are fugitive also,
And emulate, vaulted,
The lambent heat-lightning
And fire-fly’s flight.”

ROENTGEN’S X RAYS.

FIG. 232_a_.—_Living Hand. Exposure, 4 Minutes._
]

On page 507 reference will be made to certain remarkable effects observed by Mr. Crookes when the electric discharges from an induction coil are passed through very highly exhausted tubes. These phosphorescent and mechanical effects Mr. Crookes attributed to streams of “radiant matter” shot off from the _negative_ pole with immense velocities—the matter not being that of the electrode itself, but particles of the extremely rarefied residual gas, which, being comparatively few, could mostly traverse the tube in straight lines without coming into collision with their fellows, and thus a class of phenomena, different from the striated discharges in the ordinary and less highly exhausted Geissler tubes, comes into view. The emanations from the negative pole, or _cathode_, in highly rarefied gases became known as the “cathode rays,” and they began to be further examined by other observers, and more particularly in 1894 by Hittorf, and by M. Lenard, a Hungarian physicist, who found that they pass through thin plates of metal, and through wood and other substances not transparent to ordinary light. It was also observed by Lenard, and also previously by Hertz, that there are several kinds of cathode rays, which differ from each other as regards their powers of exciting phosphorescence, capability of being deflected by a magnet, and the degrees in which they are absorbed by various media. But universal attention was drawn to this subject by the announcement, at the end of 1895, of certain discoveries made by Dr. W. K. Roentgen, a professor of physics at Wurzburg. He covered a highly-exhausted Crookes’ tube with black cardboard, and found that when the discharge of a large induction coil was passed through the tube in a dark room, a piece of paper coated on one side with platino-cyanide of barium, and held near the covered tube, glowed with a brilliant fluorescence, no matter which side of the paper was turned towards the tube; and even at a distance of two yards some fluorescence was still visible. On experimenting with various bodies interposed between the covered tube and the fluorescent screen, it was found that the emanations passed through nearly every substance with more or less facility. The screen lit up when placed behind a book of a thousand pages, also behind two packs of cards. A single layer of tin-foil scarcely threw a shadow, and several thicknesses were required to produce a distinct effect. Deal boards, an inch thick, offered little resistance. A very thick plate of aluminium (6/10 inch) reduced the fluorescence, but still allowed some rays to pass. The hand held before the fluorescent screen showed a dark shadow of the bones only, with but a faint outline of their fleshy investment. Copper, silver, gold, platinum, and lead, in comparatively small thicknesses, intercept these rays. Thus a plate of lead only five hundredths of an inch thick almost stops them. Increase of thickness increases the resistance to their passage in all cases; but the comparative transparency of a body cannot be deduced from its thickness and density. Many other bodies besides platino-cyanide of barium become fluorescent under the influence of these rays, such as certain kinds of glass, Iceland spar, rock-salt, etc. Dr. Roentgen is convinced that these rays are not the cathode rays or any part of them; but as the theoretical nature of the new rays has not yet been explained, he has preferred to provisionally call them the X rays, a denomination doubtless suggested by the use of the symbol _x_ in algebra to represent unknown quantities.

The source of the X rays, Roentgen states, is at the place where the cathode rays strike the walls of the exhausted tube, and produce the most brilliant phosphorescence; but they cannot be cathode rays which have merely passed through the glass, for, contrary to what has been observed with respect to the latter, they cannot be deflected by a magnet. Nor is glass the only substance in which they can be generated, for they were obtained from an apparatus in which the cathode rays were made to impinge upon a plate of aluminium nearly one-tenth of an inch thick. Photographic dry plates are also sensitive to the X rays, and their power to pass through wood, ebonite, etc., makes the experiments of testing the opacity, or otherwise, of various objects for them quite easy. It is necessary merely to place the object on the closed cover of the dark slide, and place the whole under the vacuum tube; all the exposure, which is somewhat prolonged in most cases, may be made in ordinary light. But the light-tight boxes, in which photographic plates are packed, cannot, of course, be brought near the apparatus, as they are completely permeable to the X rays, and their whole contents may be rendered useless. The impression obtained on the photographic plate is not so much a photograph as a _shadow_ of the interposed object—a shadow more or less dense in the positive print according to the permeability of the object, and the length of the exposure. These photographic results have sometimes been called “shadowgrams,” “radiograms,” “radiographs,” “skiagraphs,” etc. The word _skiagraph_ appears the most appropriate designation. That the emanations from the phosphorescing substance on which the cathode rays impinge are entitled to be also called rays, appears from the regularity of the shadows thrown on the fluorescent screen or photographic plate; and the fact of their propagation in straight lines was proved by Dr. Roentgen obtaining a _pin-hole_ photograph of the phosphorescing part of the vacuum tube, when the latter was enveloped in black paper. Why a _pin-hole_ and not a lens was used for taking this photograph will presently appear.

FIG. 232_b_.—_Skiagraph of a Hand, by Dr. Roentgen. The Third Finger
has a Ring on it._
]

One of Dr. Roentgen’s experiments excited the attention and interest of the general public, as well as of the scientific world, in the most extraordinary degree, and though its announcement was received in some quarters with incredulity, experimenters in all parts of the world immediately set themselves at work to test the truth of the alleged discovery. Electrical apparatus of different kinds, with various adjustments, were employed, with results that were in some cases failures, in others confirmations of the German professor’s statements, and not unfrequently the variations in the conditions gave rise to increased knowledge of the phenomena generally. The experiment just alluded was one in which a dry photographic plate contained in one of the camera dark slides, now so familiar to every one, was placed (with the slide still closed by its wooden cover of nearly one quarter of an inch thick) a few inches below the Crookes’ tube, and the hand of a living-person being extended on the outside of the cover, a shadow of the bones of the hand, as if seen through the surrounding tissues, was obtained. Much popular misconception as to the powers of the “new photography” arose from want of knowledge of the process by which these strange pictures were obtained, the common notion being that these photographs were produced by some method of using a camera, and that outlines of people’s bodies and skeletons could be taken instantaneously, not only through their clothes, but through doors and walls. Much nearer the mark was the allusion of a scientific writer as to the possibility of the new process realising Dickens’ description of Marley’s ghost: “His body was transparent, so that Scrooge, observing him, and looking through his waistcoat, could see the two buttons on his coat behind.” The value of the new discovery for medical and surgical purposes was immediately recognised, and very soon its application was successfully practised.

Dr. Roentgen found that the X rays are incapable of refraction, in this respect differing from ordinary light (see pages 397 and following), and among the experiments which most impressed and astonished his auditors when he was lecturing at Potsdam on his new discovery before the Imperial Court of Germany, was one in which he showed the X rays passing in a straight line through water without undergoing refraction. The rays pass without interruption equally through the substances, whether these be coherent or in a layer of fine powder of the same thickness, and this again shows that there can be no regular reflection or refraction. Prisms and lenses, whether of glass, ebonite, or aluminium, fail to afford evidence of refractive action, hence the X rays cannot be focused like those of ordinary light, and that is why the photograph of the vacuum tube had to be taken by a pin-hole. Again, glass lenses could not be used, because this substance, so transparent to light, is particularly opaque to the X rays, and would in a great degree intercept them, while lenses of ebonite and of aluminium, which were tried, were inoperative on account of the irrefrangibility of the rays.

As to the nature of the rays themselves, Dr. Roentgen rejects the notion of their being “ultra-violet” rays, which was suggested by some. The meaning of this term is seen when it is understood that a great distance beyond the violet end of the visible spectrum there are radiations, revealed by their photographic impressions, so that the whole spectrum is really some eight times as long as the visible part. In consequence of these ultra-violet rays acting on the photographic plate, it is possible, as has long been known, to take a photograph in the dark. The eye is quite insensible to the X rays also, and although these, as we have seen, readily pass through the bodily tissues, it may be placed quite near the discharge tube, the latter being enveloped in black paper, without causing any sensation. That the new rays are in some way allied to light is the opinion held by Dr. Roentgen, and he is inclined to consider them as due to _longitudinal_ vibrations in the ether; that is, instead of the transverse waves to which light is attributed, these resemble the waves of sound, in so far that they move in the direction of propagation. This would account for the absence of any distinct refraction, or polarisation, which seems to characterise the X rays. Their connection with certain electric Maxwell-Hertz waves (see p. 541) is more problematical, as the mathematical formulæ for these admit only transverse oscillation. But on the assumption of certain conditions, due to the action of electricity, etc., on highly rarefied air, the possible existence of longitudinal vibrations has been deduced by admitting a certain variation in some of the factors of the Maxwell formulæ.

Other suggestions have been advanced in order to make the observed facts concerning the X rays fit into established theories, but so far these attempts have been unsuccessful. It would seem as if our present conceptions of light, electricity, the ether and matter, will have to be profoundly modified and enlarged in order to bring these and other recently discovered phenomena within their scope. Since the publication of Dr. Roentgen’s paper, his results have received confirmation in every quarter, and many new observations have been added, some of which seem to tend not so much to elucidate the phenomena, as to prove them even more complicated than was at first supposed. Such was the announcement in June, 1896, of the discovery of several varieties of X rays.

FIG. 232_c_.—_Metal Objects taken through Calico and Sheet of
Aluminium._
]

In the meantime, various modifications have been made in the forms of the tubes and electrodes, and divers arrangements have been used for the exciting electrical apparatus. Thus it has been found that the X rays are given off from platinum more copiously than from glass, aluminium, or any other substance, and by using a tube closed by a “platinum window,” on which the cathode rays impinge, Mr. Gifford has been able to reduce the time of exposure for obtaining a skiagraph of the bones on the hand to half a minute, whereas twenty times that period was formerly required. Another form of tube is advertised by Brady & Martin of Newcastle, with which, in conjunction with a new screen, a coil giving a 5–inch spark will, it is stated, yield a good skiagraph of the hand in _two seconds_, which appears to be the shortest time yet attained. Another firm of tube-makers, Newton & Co., London, state that their special form of tube, excited by a coil giving a 6–inch spark, and used with their fluorescent screen, “will work right through the human body, showing the heart, liver, spine, ribs, the movements of the heart and of the diaphragm, etc.” It has recently been observed that the best results are obtained when there is a certain, but as yet undefined, relation between the degree of rarefaction of the residual gas in the tube, and the intensity or frequency of the electric discharges, and that these should be accommodated to the work required. Thus, for example, if a skiagraph of the hand be attempted with an apparatus in which these factors are carried to too high a degree, the resulting X rays will pass through the bones almost as freely as through the surrounding tissues, and their shadows will therefore not appear. If, on the other hand, the contrary conditions hold, an incomplete or maybe no result will be found. This seems to explain the failures that have sometimes occurred when tubes of apparently identical construction have been used in the hands of the same, or of different, observers. Perhaps more depends also on the time of exposure. For instance, if a short exposure be given in the case of the hand, the photograph will be merely a silhouette of that member; with a little longer exposure, this will show the nails; with still longer time, the shadow of the fleshy parts begins to grow faint and the skeleton to appear. With yet more prolonged exposure only the bones will show, in their various degrees of opacity, and the shadows of these will gradually disappear as the time of exposure is increased, until at length the image will be entirely effaced. The considerable differences as to distinctness of the various tissues, which are exhibited by the published prints of hand shadows, are thus explicable.

FIG. 232_d_.—_A Skiagraph of Layers of Various Substances._
]

FIG. 233.—_Portrait of Professor Helmholtz._
]

SIGHT.

The investigations of modern science have borne rich fruit, not only by vastly extending our knowledge of the universe of things around us, but also making us acquainted with the mode in which certain agents act upon our bodily organs, and by revealing, up to a certain point, what may be termed the mechanism of that most wonderful thing—the human mind—or, at least, that part which is immediately concerned in the perceptions of an external world. Of all the physical influences which affect the human mind, those due to light are the most powerful and the most agreeable. One of the most ancient of philosophers says, in the simple words which are appropriate to the expression of an undeniable truth, “Truly the light is sweet, and a pleasant thing it is for the eyes to behold the sun.” The impression produced by light alone is a source of pleasure—a cheering influence of the highest order; and there is a special character in the pleasing effects of light, from the circumstance that they do not exhaust the sense so quickly as do even pleasurable impressions on other organs—such as sweet tastes, fragrant odours, or agreeable sounds. Sight is not liable to that satiety which soon overtakes the enjoyment of sensations arising from the other senses; it possesses, therefore, a refinement of quality of which the rest are devoid. Sight converses with its objects at a greater distance than does any other sense, and it furnishes our minds with a greater variety of ideas. Indeed, our mental imagery is most largely made up of reminiscences of visual impressions; for when the idea of anything is brought up in our minds by a word, for example, there arises, in most cases, a more or less vivid presentation of some visible appearance. Our visual impressions are also longer retained in memory or idea than any other class of sensations.

The nature of the impressions we receive through the eye is extremely varied; for we thus perceive not only the difference between light and darkness, but in the sensations of colour we have quite another class of effects, while the lustre and sparkle of polished and brilliant objects add new elements of beauty and variety. We find examples of the latter qualities in the verdant sheen of the smooth leaf, in the splendid reflections of burnished gold, in the bright radiance of glittering gems, and “in gloss of satin and glimmer of pearls.” The eye is also the organ which conveys to our minds the impressions of visible motion, with all those pleasures of exciting spectacle which enter so largely into our enjoyment of life. It likewise discriminates the forms, sizes, and distances of objects; but by a process long misunderstood, and dependent upon a set of perceptions which, although precisely those whence we derive our most fundamental notions of the objects around us, have been completely overlooked in that time-honoured enumeration of the senses which recognizes only five.

If such be the extent to which our minds are dependent upon the wonderful apparatus of the eye, it may easily be imagined what must be the comparative narrowness of mental development in those who have never enjoyed this precious sense, and the feeling of deprivation in those, who, having enjoyed it, have unfortunately lost it. Well may our sublime poet despairingly ask—

“Since light so necessary is to life,
And almost life itself—if it be true
That light is in the soul—
The all in every part: why was the sight
To such a tender ball as the eye confined,
So obvious and so easy to be quenched?”

—for he himself, in his own person, experienced this deprivation, and he thus touchingly, in his great work, laments his loss:

“Thus with the year
Seasons return; but not to me returns
Day, or the sweet approach of even or morn,
Or sight of vernal bloom, or summer’s rose,
Or flocks, or herds, or human face divine;
But cloud instead, and ever-during dark
Surround me; from the cheerful ways of men
Cut off; and for the book of knowledge fair
Presented with a universal blank
Of Nature’s works—to me expunged and rased,
And wisdom at one entrance quite shut out.”