Chapter L: J. M. Daguerre, born 1787, died 1851 (2)

Excellent photographs of the planets have also been taken by Mr. Common and others; but they are of course small, and have contributed so far, much less to our astronomical knowledge than those already mentioned. Very different are the results obtained in what, a short time ago, appeared a less promising field. The image of a so-called fixed star, in even the most powerful telescopes, presents itself as a mere luminous point, and this is the case whether the star is one of the brightest or one of the least conspicuous. The telescopic appearance is simply a more or less brilliant point. The various degrees of brightness which distinguish one star from another (_stella enim a stellâ differt in claritate_), and which the unassisted eye attributes to difference of size, led, long before the invention of telescopes, to a classification of them accordingly. The brightest stars are said to be of the 1st “magnitude,” those of the next inferior degree of brilliancy, of the 2nd “magnitude,” and so on, down to the 6th, which includes the faintest star discernible by an acute eye under favourable circumstances. But stars too faint to be thus seen came into view in the field of the telescope, and therefore those of the 7th magnitude, and beyond, are termed _telescopic_ stars, and each additional power given to the instrument brings others in view that previously were invisible. The classification has been carried down to the 18th or 20th magnitude, which expresses the limit of visibility with the most powerful telescopes yet constructed. In the methods hitherto employed for this classification, there is necessarily much that is arbitrary and vague, and it is quite common to find a different magnitude assigned to the same star by different authorities. Now the photographic plate enables the astronomer to determine the relative brightness of stars quite definitely. Everyone knows that the time required to impress an image on the sensitive plate is longer, as that image is less luminous. Hence, by finding the time required for the images of different stars to be impressed, we have a measure of their relative luminosities. Suppose the image of a group of stars is allowed to act on a plate for, say, 5 seconds, we should find only the brightest stars represented. If a second plate have double the exposure given, it would be impressed by the images of not only the brightest stars of the group, but also by those of the next degree of brilliancy; and a third plate exposed for 20 seconds would show more stars than the two former exposures. So that plate after plate might be exposed under the same group for successively longer and longer intervals indefinitely. Exposures extending over hours have been made, notably by Mr. Common in England, and by Mr. Gill at the Cape of Good Hope, showing not only how magnitude may be determined to any extent, and the heavens most accurately mapped out, but with this very remarkable result:—_thousands of stars, invisible even in the most powerful telescopes, are portrayed in the photographs_. Let us consider for a moment the significance of this fact with regard to the new space-exploring powers it has placed in the hands of science. The number of stars visible to the unassisted eye in the whole expanse of the heavens has been variously estimated, but the figures usually given lie between 3,000 and 4,000, and the highest estimate for the most acute eyesight, under the most favourable atmospheric conditions, places the limit at 5,000. The brightest star in the heavens is Sirius, and Sir. J. Herschel ascertained that its light is about 324, that of an average star of the 6th magnitude. Taking the average luminosities of stars of the first six magnitudes, Sir W. Herschel, from his own observations, represents their relative brightness by the following figures: 100; 25; 12; 6; 2; 1. The different degrees of brightness seen is, probably, due to the following three causes, combined in various proportions: (1) the different sizes of these luminaries themselves; (2) differences in their intrinsic luminosity; and, (3) differences in their distances from us. And it is also extremely probable that the last is generally by far the largest factor of the three. It has been found by photometrical experiments, that the light we receive from the sun is 20,000,000,000 (twenty thousand million) times more than that of Sirius. If we suppose Sirius to be in reality only as large and as bright as our sun, it follows that its distance from us must be no less than 13,433,000,000,000 miles. The distance of stars of the 16th magnitude has been estimated to be such that their light—travelling at the rate of 185,000 miles per second—takes between five and six thousand years to reach us. For a long time no sensible parallax could be discovered in any of the fixed stars; that is, no change in their positions was discernible when viewed from points 183,000,000 miles apart, namely from the extremities of a diameter of the earth’s orbit. In other words, if we suppose the line of the length just mentioned to form the base of a triangle, having a star at its vertex, the angle formed by the sides is so small that the most refined instruments failed to measure it. In recent times, however, the parallax of a few stars—about a dozen or so—has been detected and approximately measured. The greatest observed parallax belongs to in α the constellation of the Centaur, a star of the first magnitude, 30° from the south pole of the heavens, and of this the parallax amounts to but a little more than nine-tenths of a second of angular measurement, corresponding with a distance of nearly 20,000,000,000,000 miles, a space which takes light 3½ years to pass over. This star is, therefore, believed to be the nearest of any to our system. The smallest parallax that has been measured in any of these few stars is a fraction of a second of angle corresponding with a distance twenty times greater than the other, and requiring seventy years for light to traverse it. Now, as the photographic plate shows us stars of magnitudes indefinitely smaller even than the telescopic sixteenth, we cannot but marvel at the manner in which the light travelling from these suns in the immeasurable depths of space, and taking untold thousands (nay, millions, it may be) of years in its journey is yet able so to agitate the atoms of our silver compounds that images of things that will themselves, probably, never be seen by mortal eyes are presented to our view. A circumstance requiring explanation will occur to the reader’s mind in connection with stellar photography; and that is, how does it happen that, if the image of a star is a mere point, it nevertheless impresses the plate as a visible dot? It is probably because the point is a centre whence the photographic influence radiates laterally on the plate to a small but yet sensible distance.

Among the cosmic objects presented to our observation there are none more fully charged with interest and instruction than the _Nebulæ_. These are faintly luminous patches, in some few cases visible to the naked eye, but for the most part telescopic. The milky way, which extends round the celestial sphere, is a very conspicuous phenomenon of the same kind. A few other hazy, cloudlike patches are seen in various parts of the heavens, visible on a clear moonless night when the eye is directed towards the proper quarter. The well known group of the Pleiades sometimes presents this appearance, but most persons are able by the unassisted vision to discern in it a group of six stars at least, and an opera-glass or ordinary hand telescope easily resolves the object into a cluster of 20 or 30 distinct stars. Telescopes of higher powers bring more stars into view, and as many as 118 have been counted in the group. There are several other groups of this kind perceptible to the naked eyes merely as diffused patches of light, but resolvable by the telescope into thickly clustered groups of minute stars; but in many of the resolvable nebulæ the separate stars appear spread on a back-ground of diffused luminosity. Again, there are other nebulæ which telescopes of the highest powers we possess fail to resolve at all. Not only has the photographic method shown stellar components of some of these last, but it has depicted the form of nebulæ never seen at all, and whose existence was previously unknown and unsuspected. For example, the photograph has revealed the existence of a back-ground of nebulous patches to the stars of the Pleiades—a thing that had never before been suspected, although the group has been repeatedly observed by the most powerful telescopes. Those who are at all acquainted with astronomy, will understand the significance of this discovery for the science. The results already obtained afford a marvellous support to the famous speculation known as the nebular hypothesis. And as the forms of these objects are accurately shown for us by their own light, changes in their appearance may thus be detected as time goes on which may serve to lift the above named theory into the region of demonstrated truth. The nebulæ which neither telescope nor camera can resolve are such as the spectroscope proves to be masses of glowing gas or vapour.

It has been already mentioned that the light from these immeasurably distant stars and nebulæ is so faint that the most sensitive photographic plates have to be exposed for hours. This would be a matter of no difficulty if the clockwork mechanism by which the apparatus is made to follow the apparent motion of the heavens could be constructed with absolute perfection. But as this is not obtainable, even with the most careful workmanship, and the smallest jar or irregularity would distort and confuse the images, this source of disturbance is eliminated in the following manner: attached to the photographing apparatus and driven with it is a telescope, provided with cross wires, and through this an observer views some star during the whole period of the exposure, his business being to keep the image of the star accurately on the cross wire, which he is enabled to do by having the means of slightly modifying the movement of the clockwork. In the Paris Exhibition of 1889 were shown many very fine large photographic prints of nebulæ (notably of great nebula in Orion), which have recently been obtained in this manner, and those nebulæ that had been photographically resolved had the stellar components marked with wonderful distinctness. Comets and meteorites have been photographed, and even the _aurora borealis_ and the lightning’s path have been brought within the camera’s ken.

Space would fail us to describe the many applications now found for photography in microscopy, in medicine and surgery, in anthropology, in commerce, and in the arts. It is obvious also from the improvements that are continually made, that many of these applications have not yet received their full developments. Photography has been enlisted into the service of the army and navy, and regular courses of instruction in the art are given in their training schools. A well equipped photographic waggon now accompanies every army corps, and in almost every ship of war, some proficient operator is to be found. By an ingenious combination of photography, aerostatics and electricity, it is possible to obtain with perfect safety accurate information of the disposition of an enemy’s forces and fortifications. A small captive balloon is sent up, to which is attached a camera. At a height of a few hundred yards, the balloon is practically safe from any projectiles, and in its cable are interwoven two electric wires by which currents are conveyed to electro-magnets, which produce all the movements required for any number of exposures. Jurisprudence has found its account in recognizing the art, for the photograph is received in evidence for proving identity, etc. The administration of the criminal law takes advantage of the art to secure the likeness of prisoners for future identification, and the modern instantaneous process renders unnecessary the subjects’ concurrence with the operation. Again, if the “hue and cry” has to be raised for an individual “wanted” for any offence, and a photographic likeness of him is procurable, thousands of copies can be made of it in a few hours, by night as easily as by day, and distributed to every police station in the whole country.

Modern processes now enable us to obtain prints from negatives in as many seconds as a few years ago hours were required, and this by artificial light. A process of printing lately introduced and yielding artistic results which deserve to find more general favour, is that called the _platinotype_. Instead of the ordinary print produced on lightly glazed paper by the reduction of silver compounds, and of questionable permanency, the image is formed in the paper by metallic platinum, the most changeless of all possible substances under ordinary influences. The pictures are of a rich velvety black, with soft gradations, and the surface is without glaze or glare. The print has, in fact, the appearance and all the best qualities of the most highly finished mezzotint engraving, combined with the minute fidelity characteristic of the photograph. The problem of producing a photograph in colours, permanently showing nature’s tints in all their gradations, has still a great fascination for some experimenters, and startling announcements are made from time to time of some discovery in this direction. It does not appear, however, that any success has really been arrived at, beyond the results long ago obtained by Becquerel as described on page 614; and, indeed, as our knowledge of the science of the subject increases, the less likely does the possibility of photographing colours appear. It is, however, never safe to lay down the limits of discovery in science.[14] Note that precisely in the matter of rendering colour even in its due gradation of tone or luminous intensity, the photograph is quite untruthful. Everybody has noticed how unnaturally dark and heavy the foliage of trees appears in the prints; if we suppose a lady in a blue dress, with yellow trimmings, to sit for her portrait, the photograph will show her in a white dress with black trimmings; a sitter with light yellow or auburn hair will appear of quite a dark complexion; if you photograph a lemon and a plum together, the latter will probably come out lighter than the former; or if a daffodil be the subject, the flower will be drawn in tones much darker than the leaves. This incorrectness of tone relations can, however, be greatly lessened by the device of reducing the quantity of the blue rays, by interposing a piece of optically plane yellow-tinted glass, by using the sensitive plates tinted with certain coal-tar dyes, which are now prepared and sold under the name of “ortho-chromatic plates,” or by both methods combined.

Footnote 14:

See page 630.

If any illustration were needed of the great popularity now attained by the practice of photography, reference might be made to the large number of periodicals devoted to the subject, and appearing weekly, fortnightly, quarterly or annually, in every civilised country, and also to the multitudes of societies that have been formed for the promotion of the art. In Great Britain alone there are now at least 150 such societies in active operation, and they are correspondingly numerous elsewhere. If, when we consider all that has been accomplished up to the present time, with the jubilee year of photography scarcely passed, and observe the increasing numbers of its cultivators guided by the explanations of its phenomena that science is beginning to furnish, we can expect a corresponding progress in the next fifty years, then the centenary may be reached with a roll of achievements that could we know them now we should think marvellous.

As already remarked elsewhere, the practical side of photography has outstripped the theoretical one, for so far its progress has been much less indebted for processes and technic to the direct guidance of science than almost any other of our Nineteenth Century acquisitions, such as telegraphy, electric lighting, etc. The materials employed, and the mode of manipulation, have certainly _not_ been deduced from previous knowledge of the nature of light or from the laws of chemistry, although when, by repeated trials and happy guesses, the right direction had been found, the field into which it led could be more easily explored under the direction of chemistry and physics. But even yet the fundamental principle, or the precise nature of the action of light on certain compounds, has not been definitely made out, and although some theories on the subject have been proposed, no one has been generally accepted as an adequate explanation of the known facts, and still less have any quantitative relations been established for these actions. The photographer cannot compose a formula for the composition of his emulsions and developers from assured data like those that enable the chemist to weigh out with accuracy the constituents that go to produce a required compound.

The attainment of permanency in its products, which, by several processes, photography can now boast of, is one of its triumphs, and will tend greatly to enlarge the sphere of its utility. For example, we have a public institution, known as the National Portrait Gallery, in which it is sought to gather together and preserve the likenesses of the most eminent Englishmen, and presentments of such of far less fidelity than photographic portraits are eagerly sought after. It has been suggested that something like a National Gallery of _permanent photographic portraits_ of the chief men of their time would be a fitting and acceptable legacy to the public of the future. This idea has much to recommend it, particularly as authentic likenesses would thus be secured for the nation beyond the chance of loss.

Photography has been applied in preparing blocks in relief for printing along with letterpress in the same way as woodcut blocks. The process has the great advantage of producing in a wonderfully short time a perfect facsimile of the artist’s drawing without the intervention of any engraver. A plate of zinc, brass, or copper, coated with a dried film of bichromated albumen, is exposed to light under the transparent negative of a drawing in pure line, that is, one having in it only lines of uniform colour throughout. The parts of the film reached by the light, which correspond with the lines of the original design, are rendered insoluble, while the rest can readily be removed by water. These unprotected parts have then to be removed by the action of acids, but these are used alternately with the application to the plate of certain compositions, the purpose of which is to prevent lateral erosion of the lines in relief before the requisite depth of the metal has been removed. Fig. 147_f_ is the reproduction of a pen-and-ink sketch by this or some similar process. But nature and the ordinary photograph show us graduated tones which ordinary printers’ ink cannot really reproduce, inasmuch as it is incapable of gradation, and can give the _effect_ of gradation only by such devices as are mentioned on page 642 (last sentence). Now, the photograph cannot yield a printing-block until its continuous tones are broken up into lines or dots. Not a few methods of doing this have been contrived, but that which is by far the most commonly used, and is most successfully practised on the commercial scale, is simple in principle, although in actual working it calls for much experience and skill. The negative is taken upon a wet collodion plate, in front of which, within the camera, and at a very short distance (say 1/30th inch) from the film, is a transparent _screen_, bearing two sets of parallel opaque lines at right angles to each other. These lines are mechanically ruled with the utmost regularity, and are separated by only very small intervals. There may be from 80 to 200 of them in the space of one inch, according to the class of work required. The effect of this is that the light reaches the photographic film through a series of minute transparent squares, the sides of which may be only from the 1/140th to the 1/400th of an inch in length. Now it is found that the brighter lights from the original positive, after passing these small apertures, spread so as to more or less cover the opposite parts of the negative, while the feebler lights, from the shades of the original, impress the plate to a less degree, the developed image in these showing, perhaps, merely a small dot or, in the very darkest parts, a blank. In this way, then, may the photographic negative be obtained with a granulated texture following in graduation the tones of the original. After this, the rest is easy, for the process of exposing a metal plate, coated with a sensitive film under the negative, and of etching it with acids, etc., is essentially the same as in the foregoing. Such is the _half-tone process_, which is now so largely superseding wood and other engraving. It is unnecessary to describe technical details here, such as the employment of _bitumen of Judæa_ as the coating for the metal plate, or how the image must be reflected into the lens from a mirror to avoid a reversal in the final print, etc. There are endless modifications of the processes briefly mentioned above, and some of these are guarded as valuable trade secrets. Several of the illustrations in this work are prepared by the half-tone process, of which plates I., IV., V., etc., are examples, and they should be examined with a strong lens, in order that the different rendering of the light and the dark parts may be compared.

_PHOTOGRAPHY IN COLOURS._

It is the statement as to the futility of assigning limits to scientific discovery that has been justified by facts. The preceding edition of this work was not long in the hands of its readers before the solution of the problem of photography in colours was announced from Paris, where, at the close of 1890, the physicist M. Lippmann had succeeded in photographing the solar spectrum in its natural colours, and at the beginning of 1891, he was able to exhibit at the Academy of Science untouched photographs of a stained glass window in three colours, of a dish of oranges and red flowers, and of a gorgeously coloured parrot, all in their natural tints. The method employed had no apparent relation to that of Becquerel, but was of the simplest, and, moreover, one which any reader who has followed the first few pages of our section on the “Causes of Light and Colours” will have little difficulty in completely understanding, if he has devoted a little attention to Fresnel’s interference experiment. M. Lippmann took a photographic plate, coated to a greater depth than usual with a gelatine film containing the sensitive salts of silver, and in the camera this plate was exposed with the glass towards the lens, while at the other side of the film was a metallic reflecting surface, namely, quicksilver. Supposing a ray of red light to enter the glass and traverse the film, it would be reflected from the metallic surface, and would meet the direct ray within the substance of the film, with a difference of length of path that would produce the interferences already described, and so give rise to alternate lines or bands of darkness and brightness. It would, of course, be in the lines of maximum brightness that the silver would be first deposited by the photographic action, and these microscopically fine lines or striæ of silver would give back, from ordinary light, a colour corresponding to the waves of red light that produced them. Similarly with the other colours. Anyone may observe the production of colour from ordinary white light in the iridescent tints of mother-of-pearl, where the effects are due to the varying distances of fine edges of the layers of the substance. If an impression is taken from a piece of mother-of-pearl by solid paraffin, or by white wax, or even by common red sealing-wax, the colours will seem to be adhering to the impression, but the operation may be repeated times without number. It is the distance apart of the lines or striæ that determinates the colour, and this is always some definite multiple of the wave lengths, given on p. 411, for the various colours. M. Lippmann’s products are true colour photographs, and they form a new and elegant experimental demonstration of the doctrine of luminiferous undulations.

The colour effects of nature have also been reproduced by taking photographs of the same scene through coloured glass. Thus a screen of yellow glass will intercept the blue and the red rays, and the sensitive film will be impressed with images of objects containing yellow rays only, and that in proportion to the quantity of these rays that enter into any given tint. Similarly with images taken through red and blue glasses. The positives from these partial images being projected by three optical lanterns on the same space on a screen, and each being coloured by passing through tinted glasses like the original, the superposed images thus combined give a very lively impression of the natural colours in all their gradations.

Among the many processes for reproducing photographs by non-photographic processes, some have been more or less successfully combined with colour printing. Some of these productions are very effective, and are more attractive to many persons than the monochromatic tints of ordinary photographs.

FIG. 312.—_Portrait of Aloysius Senefelder._
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PRINTING PROCESSES.

As it is beyond contradiction that printing is one of those inventions which have most influenced the progress of mankind, so it will be admitted that certain modern processes, by greatly facilitating the operations, and vastly extending the resources, of the art, possess an interest and importance surpassed by few of the subjects we have discussed. In a former article the reader has been made acquainted with the steam printing-press and other applications of machinery by which the impressions of a form of type, or of a pattern, can be rapidly multiplied. Here we have to describe some ingenious methods of preparing the forms or originals for letterpress and other printing, and certain beautiful processes for multiplying drawings, engravings, and pictures.

_STEREOTYPING._

This term is applied to the process of obtaining the impression of a form of movable types, or of a woodcut, on a plate of metal which can be printed from. These plates, after the required number of copies have been printed, can be stored away; and they are ready for use whenever another issue of the work is required. When the pages that are to be stereotyped have been set up in ordinary type, there are several methods by which the stereotype plates may be obtained from them; or rather, there are several materials used to form the matrix or mould in which the metal is cast. When plaster of Paris is used, the form is first slightly oiled, to prevent adhesion of the plaster; a thin mixture of plaster and water is then poured upon the form, which is surrounded by a raised rim, to retain the plaster. The thin plaster is carefully led into all the recesses of the type, and then some thicker material is poured on. The plaster soon sets, and is lifted off the type, and, after drying, is ready to receive the molten metal of which the stereotype plate is formed. This metal is an easily fusible alloy of lead, antimony, and other metals, which takes the form of the mould with great accuracy, and is, when solid, sufficiently hard to print from.

FIG. 313.—_Press for Stereotyping by Clay Process._
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Another plan is to make use of prepared clay, spread upon an iron plate, for the formation of the mould. The face of the type is brushed with benzine, the plate with the clay is laid upon it, and pressure is applied. The whole is then dried in a slow oven, and the clay, when detached from the type, is ready to form the mould. The advantages of the clay process are that the type does not require to be afterwards cleaned from oil, and that the material does not fill up the deeper spaces of the form, so that a thinner stratum of metal suffices to form the stereotype plate.

A third mode of obtaining the mould has been already mentioned in connection with the Walter Printing Press (page 313), in the working of which the _papier maché_ process is ingeniously made to supply the curved stereotype plates for the cylinders. This process is also largely used for other newspaper presses, and sometimes for bookwork, as it forms an invaluable means of expeditiously obtaining a number of stereotype plates from the movable types. This production of a number of similar forms makes it possible to strike off a very large number of copies in a short time, for many presses can be employed simultaneously. For the paper process a number of sheets of tissue-paper are pasted together, and the moist paper is laid upon the form; then the operator, by light strokes of a brush, beats down the paper into the hollows of the type, beginning at the centre of the page, and going towards the margins. A sheet of stout unsized paper, called “plate paper,” constitutes the upper layer; and when the whole has been well beaten down upon the type, pressure is applied by means of a screw acting upon a plate of iron covering the whole. In this condition a gentle heat, produced by steam, is made to completely dry and harden the paper matrix, which is very soon fit to be used for casting the metal. The apparatus for this purpose consists of a hollow iron table, within which steam is made to circulate. On this the form is placed, and the platen is pressed down upon it by means of a screw. In many cases the platen also is heated by steam, to accelerate still further the drying of the matrix, which is effected in about four or five minutes. One paper matrix, by careful use, will serve for the production of a series of casts without receiving any damage from the molten metal, as this is fusible at a low temperature.

The mould for casting flat stereotype plates from the paper matrix is made of iron, and has parallel surfaces, which admit of being so adjusted that the thickness required in the plates may be obtained very nearly. The paper matrix is laid on the horizontal iron bed of the mould; gauge-bars are adjusted, which retain it in its position; and then the second plate is folded down—the distance between that and the paper being determined by the gauge-bars. The cover is secured by clamping-screws, and then the mould is turned upright to receive the metal, which is removed, when solid, after the mould has been turned back into its horizontal position.

However the stereotype plates have been produced, it is necessary accurately to adjust their thickness by planing off some of the material from the back. The edges have also to be cut and trimmed to the exact dimensions required by the press. Various machines have been devised for effecting all these operations with accuracy and dispatch. The plates are afterwards mounted on wooden or metal blocks to bring them to the height of ordinary type.

A fourth method of producing plates for the same purpose as the stereotype plates already described is by _electrotyping_. This method appears to have been introduced as early as 1840, but the first results were not without imperfections. Now, however, this plan is almost universally applied to bookwork and woodcut illustrations. Many of our popular illustrated periodicals have so large a circulation that the wooden blocks would necessarily be spoiled by being used in steam presses long before they had yielded the required number of impressions; and the method has also the great advantage of securing the original engraving from the chance of accidental damage, by which a block is sometimes irretrievably injured. Hence woodcut illustrations are now always printed from electrotype copies of the engraved blocks, whether the work itself be printed from movable type or not. But the electrotype or stereotype process is always resorted to in the case of a work, whether illustrated or not, when it is foreseen that a re-issue will be demanded. These processes are also of great advantage to the practical printer, because when the pages set up in type have received their final corrections, he can take the casts, and then the type may be distributed—that is, returned to the cases ready for the compositors to use for other work.

The electrotype process is almost as simple as those for producing stereotype plates by casting, and its productions excel these by their great durability and extreme exactness of reproduction. We may take it for granted that the reader is familiar with the fact that ordinary letterpress characters and woodcuts are printed from forms, in which the black portions are in _relief_. For woodcuts the artist makes the drawing, in reversed position, on a block of finely-grained boxwood, in which the fibres of the wood are perpendicular to the surface. The engraver hollows out all the parts which in the impression remain white, while all the parts which are to receive the ink and produce the black parts of the impression must be left at the original level. The wooden blocks thus engraved would serve to produce a certain number of impressions, which could be taken off by careful hand-printing without perceptible damage to the block. But the pressure necessary for printing inevitably crushes the projecting parts of the block; and the impressions, after a certain number, lose their sharpness. This is especially the case in machine printing; but not only does the electrotype cast present a surface capable of bearing hard usage much better than those of the hardest wood, but even if the number of impressions required should wear out the metal plate, it can easily be replaced by another cast from the original block.

The mould which serves to give the electrotype cast may be made either of gutta-percha softened by a gentle heat and applied to the wood, or of wax. In either case a powerful pressure is applied, in order to force the yielding substance to take the forms of the engraved block or of the metal type. Wax is now generally preferred; the yellow wax used for this purpose is melted, and poured into a shallow pan; when it has become solid, it is sprinkled over with finely-powdered pure blacklead, which is brushed over the surface, and then the excess is removed by blowing with bellows made for the purpose. Thus prepared, the wax is placed over the type-form or wooden block in a powerful press, sometimes worked by hydraulic power; but more frequently a _toggle_ press is employed, in which the pressure is given by a screw and crank-wheel acting on two _elbow joints_, or _toggles_. For the information of non-mechanical readers it may be stated that a “toggle” consists of two bars jointed together, and placed _nearly_ in a straight line: when a pressure is applied to the joint, tending to bring the rods still more nearly into a straight line, their extremities are thrust apart with a great force, which increases indefinitely as the rectilinear position is approached. In the electrotyper’s press there are two toggles constructed of very broad bars, or rather thick plates, for they have nearly the width of the bed of the press. With this machine a very powerful and regular pressure is applied; and the wax in a few minutes takes a sharp impression, embracing all the most delicate details of the work, and becomes at the same time very hard. The impression, of course, has hollows corresponding to the projections of the wooden block or type-form, and _vice versâ_. The face of the wax mould is now very carefully and completely blackleaded, a soft brush being used in the process. It is then placed in the solution of sulphate of copper, and the blacklead receives a deposit of copper, in the manner explained in a former page (498). In about forty or fifty hours a firm, compact deposit, about as thick as the finger-nail, covers the blackleaded surface, forming a perfect reproduction of even the most minute details of the engraved block or letterpress form.

The next operation has for its object the removal of the thin shell of copper from the wax. This is effected by exposing the mould to a gentle heat by immersing it in hot water, or by placing it on a hollow iron table which is heated by steam. The wax is run off into a proper receptacle for future use, and any portion adhering to the copper is removed by the action of naphtha or of a solution of potash. The thin copper shell is then tinned on the back, and an alloy of lead with some tin and antimony, forming the _backing metal_, is poured on it, to the depth of about one-eighth of an inch. When this has become solid the backing is planed, so that the compound plate may have a certain regular thickness, and that the back surface may be parallel to the face. The edges are cut by a circular saw and trimmed by machine-tools, and the plate is rendered perfectly even, and adjusted with the greatest possible exactness to the required thickness. It is prepared for the press by being screwed down upon a block of wood of a certain thickness, so that the face of the plate may have the same height as common type, the screws passing through the margin or other hollow parts of the face of the cast. No more enduring surface than the copper of these electrotype casts, backed up by the hard alloy, has yet been discovered.

_LITHOGRAPHY._

To Aloysius Senefelder, a musician attached to one of the theatres in Munich, whose portrait appears at the head of this article, is due the invention of the art of lithography. It is said he used to arrange his musical compositions on a kind of slates, formed of flakes of the limestone which is found in the neighbourhood of Munich. One day a memorandum which he had made in this manner happened to fall into a slop-bucket full of greasy water; on withdrawing the piece of stone, he noticed with surprise that the grease had attached itself to the characters, while the rest of the stone remained quite clean. Such an incident might have happened to each one of a thousand men, and its significance might not be perceived; but it suggested great possibilities to Senefelder, who, applying himself for some years with ingenuity and perseverance to experiments with the Munich limestone, became, in the year 1800, the inventor of a new art. Though he was no chemist, and was unskilled in mechanics and in drawing, yet within four years from his first observation he had succeeded in finding the proper materials for his crayons and the appropriate acids for acting on the stone, in contriving a suitable press for taking the impressions, and in producing samples of lithographic work in various styles of art. He endeavoured to keep his processes secret, and having obtained the exclusive right of exercising his invention in his own country, he attempted to carry on all the operations himself. Little by little, however, the general nature of the process became known, and although the details were jealously concealed, ingenious persons in France and elsewhere, by force of experiment, succeeded in re-inventing the art for themselves, and Senefelder never profited by his invention as he should have done.

The first lithographic press in London was established by Mr. Hullmandel in 1810. The value of lithography as a means of multiplying works of art was soon afterwards proved by the publication of a magnificent series of picturesque delineations of the quaint architecture of the old towns of Flanders and Germany, drawn on the stone by Samuel Prout. The late Mr. J. D. Harding largely contributed to the popularity of lithography by the landscapes which he drew on the stone, and thus placed in the hands of every one, prints in which all the freedom and force of the artist’s work were secured. The French designers excel in fine-art lithography, and many beautiful productions of their crayons have been published in every department of pictorial illustration.

The best lithographic stones come from Germany; but for some kinds of work stones from other localities are used, on account of their less cost. Thus, in England, a stone yielded by the white lias formation near Bath has been found to possess the requisite qualities. The stones for lithography are prepared in much the same way as slabs of marble are polished; that is to say, by rubbing one slab against another with sand and water. When the stones have thus been brought to a plane surface, they are finished according to the purpose for which they are intended. If they are intended to receive written characters, they are polished to a very smooth surface by means of pumice-stone. But if they are to take drawings, then a certain uniform grain is given by means of finely-sifted sand, the operation being performed in a similar manner to that in which the stones are dressed, only pressure is not applied to the upper stone. The stones, after being washed and dried, are carefully covered on their prepared surfaces with thin paper, and are sent out for use.

When the stone is employed to reproduce written characters, or drawings imitating those done with a pen, _lithographic ink_ is made use of with an ordinary pen, a ruling-pen, a fine brush, or a pen which the lithographer makes for the occasion out of thin metallic plates. The composition of the ink varies much: the usual ingredients are wax, gum-mastic, gum-lac, soap, and lampblack. This composition forms a solid, which is rubbed down with water to a thick liquid when required for use. The characters have, of course, to be written on the stone in a reversed position, and the lithographer acquires the habit of doing this with neatness and dexterity. He is provided with a looking-glass for viewing his work, in order to see the effect which will be given by the impression, for the looking-glass shows the characters in their usual position, just as the image of ordinary writing seen in it is reversed, showing, in fact, the very appearance the characters present on the stone. For a drawing, a _lithographic crayon_ is used, made of wax, soap, grease, lampblack, and other ingredients. With this the drawing is made on the stone exactly as on paper, save the necessary reversals.

When the design has been placed on the stone, a liquid containing nitric acid and gum is poured over it. This liquid acts on all the parts of the stone not protected by the ink or crayon: they are thus rendered incapable of receiving printing-ink, while the protected parts have the impression more strongly fixed; for when the stone has been well washed with water, and turpentine has afterwards been applied, so that all the matter used in marking the design is dissolved away, the seemingly obliterated characters reappear when—after the stone has been lightly wiped with a damp sponge—the roller charged with printer’s ink is applied. The ink is taken up by the stone only at those places which have not been acted on by the acid. The impression is obtained by laying a sheet of damp paper on the inked stone and applying pressure by means of a roller, under which the stone passes. The stone is moistened with water after each impression before the inking-roller is again applied.

The lithographic stone, like other originals used in printing, is liable to deteriorate when large numbers of impressions are taken from it. This would be a serious drawback in lithography, but for a method of renewing the impression, which renders it unnecessary for the artist to retouch his work. This is the process of _transferring_, which is practised by the aid of a certain kind of paper specially prepared by a coating of paste. On this a proof is taken from the original drawing on the stone, and the still moist sheet is then applied to another stone, with the face downwards, and passed under the press. The effect of the pressure is to cause the adherence of the layer of paste to the stone; and when the paper has been thoroughly wetted at the back, it may be removed, leaving the paste still adhering to the stone, with the impression beneath it. When water is applied, the paste is washed off, while the ink of the impression remains attached to the stone, there reproducing the design drawn on the first stone. The transferred design is treated in exactly the same manner as the original drawing, acid being poured over the stone, &c., and the impressions obtained by the same method of successively sponging, inking, and pressing. The transferred drawing may be made to yield another transfer, and so on indefinitely; but when a large number of impressions from one design are required, it is usual to make at once from the original as many transfers to separate stones as will yield the required number of impressions without deterioration. In this way as many as 70,000 copies have been taken from a single drawing without their showing any marked difference in the character of the impressions.

The transfer process is also applied to place on the stone characters which have been written with a pen in the ordinary manner on prepared paper. In this way a person’s handwriting is so accurately reproduced in the impressions that it is often very difficult to detect the interposition of the lithographic stone, and the impression often passes as the immediate production of the writer’s pen. It is obvious that drawings etched with the pen on transfer-paper can be printed from in the same manner. And line engravings, which have been originally produced by cutting hollow lines on polished plates of copper, can be printed lithographically by transferring an impression to the stone. By transfer also the impressions of raised types or of woodcuts can be printed from the stone when desirable.

A beautiful and important application of lithography to the reproduction of pictures in colours has been so successfully carried out that a new branch of the art, termed _chromo-lithography_, now gives facsimiles of water-colour drawings and of paintings in oil. The copies of water-colour drawings especially are remarkable for their artistic qualities, and it is undeniable that these cheap reproductions of good paintings have done much to extend the knowledge of art. It is not contended that a chromo-lithograph, for example, after one of old William Hunt’s rustic figures, or birds’ nests with banks of primroses, can possess the wonderful refinement of the original; but it will nevertheless convey much of the artist’s sentiment. Such transcripts of the works of our best artists adorn the homes of thousands who have never perhaps had the opportunity of even seeing the painter’s original handiwork. In many a remote settlement in distant colonies, as in many an English home, the chromo-lithograph is the brightest of the household art treasures.

The principle of chromo-lithography consists in printing on the same paper with inks of various colours from different stones successively, so as to produce, by the juxtaposition and superposition of the various tints, the effect of a coloured drawing or painting. The artistic effects of the best chromo-lithographs require a great number of printings for their production, in some cases as many as twenty different stones being employed. The stones and colours for such productions require true artists to prepare them, persons who can thoroughly understand and enter into the spirit of the original work. The first operation consists in the preparation of a faithful but spirited outline of the original, etched on transfer-paper, from which the outline is placed on a lithographic stone. This sketch we have called an outline, but it is in reality something more; for it should suggest all the markings and limits of tints which belong to the original. This first sketch has some points marked on the margin by dots or crosses, which serve to secure true register in the subsequent processes; that is, the impressions of the successive tints are so placed on the press that these points coincide in each impression.

From the first stone as many impressions of the sketch are transferred in light ink to other stones as there are colours required in the reproduction. To each colour a special stone is assigned, on which the lithographer, guided by the slight impression of the sketch, draws with the ordinary black crayon the form which that colour is to produce on the paper. Much artistic skill and judgment are required to do this in such a manner as to obtain a clear and harmonious final result. The gradations of the colours, and their blendings by superposition, must be carefully regarded. When the form and limits of each colour have been skilfully laid down upon its own stone, the surface is acted on by the acid, it is washed, the ink is dissolved off by turpentine, the stone is sponged, and the roller charged with ink of the appropriate tint is passed over it. The ink, as before, adheres only to the parts over which the crayon has passed, and an impression may be drawn off. Each of the other stones is similarly treated, and when the whole are ready, a proof is taken by giving the same sheet of paper the whole series of impressions in their proper order and colours, with the greatest possible accuracy of register. If any alterations appear desirable, they are made accordingly, by aid of certain devices which need not be here described, and when a satisfactory result has been obtained, the printing of the whole series of impressions is proceeded with. When the number of these is very large, transfers of each stone are taken as in ordinary lithography, only with certain extra precautions for obtaining precision in the register.

The brilliant effects produced by using gold and silver in lithography are obtained by using a kind of varnish, instead of coloured ink, for printing those parts where the metal is to appear. When this varnish has acquired a certain stickiness by partial drying, powdered gold or silver is applied, and this attaches itself only to the varnish; when the sheet is dry it is passed under a burnished steel roller, the pressure of which imparts a brilliant lustre to the metal.

A method of colour-printing, in some respects resembling that of chromo-lithography, is practised by printing in variously coloured inks from a series of wooden blocks. This admits of far greater expedition in working off the impressions than the process with stones. The gradations of the coloured inks and powdered tints are produced in the same manner as those of ordinary woodcuts in black and white; and when the colours are well chosen, and care is taken to secure the accurate superposition of the impressions, very pleasing effects can be produced by this means. The coloured prints which are from time to time issued as supplements to the “Illustrated London News” are produced by this process, and are no doubt well known to the reader. Our plate of spectra, No. XVII., is an example of another method of printing in colours.

_OTHER PROCESSES._

In recent times a great number of printing processes have been devised, but only a few have found their way into practical use, and some of these have scarcely been so extensively applied as their merits appear to deserve: either because the public demand has been insufficient to bring these inventions into common use, or the cost of working them has been too great. There is no doubt of their scientific success, whatever may be their commercial value as competing with cheaper and readier methods. We shall first describe the plan which has been termed _Nature Printing_.