Chapter VI (1)

MISCELLANEOUS REMARKS IN RELATION TO TELESCOPES.

The following remarks, chiefly in regard to the manner of using telescopes, may perhaps be useful to young observers, who are not much accustomed to the mode of managing these instruments.

1. _Adjustments requisite to be attended to in the use of telescopes._ When near objects are viewed with a considerable magnifying power, the eye-tube requires to be removed farther from the object-glass than when very distant objects are contemplated. When the telescope is adjusted for an object, 6, 8, or 10 miles distant, a very considerable alteration in the adjustment is requisite in order to see distinctly an object at the distance of two or three hundred yards, especially if the instrument is furnished with a high magnifying power. In this last case, the eye-tube requires to be drawn out to a considerable distance beyond the focus for parallel rays. I have found that, in a telescope which magnifies 70 times, when adjusted for an object at the distance of two miles, the adjustment requires to be altered fully one inch in order to perceive distinctly an object at the distance of two or three hundred yards; that is, the tube must be drawn, in this case, an inch farther from the object-glass, and pushed in the same extent, when we wish to view an object at the distance of two or three miles. These adjustments are made, in pocket perspectives, by gently sliding the eye-tube in or out, by giving it a gentle circular or spiral motion till the object appear distinct. In using telescopes which are held in the hand, the best plan is to draw all the tubes out to their full length, and then, looking at the object, with the left hand supporting the main tube near the object-glass, and the right supporting the eye-tube--gently and gradually push in the eye-piece till distinct vision be obtained. In Gregorian reflecting telescopes this adjustment is made by means of a screw connected with the small speculum; and in large achromatics, by means of a rack and pinion connected with the eye-tube. When the magnifying power of a telescope is comparatively small, the eye-tube requires to be altered only a very little.

There is another adjustment requisite to be attended to, in order to adapt the telescope to the eyes of different persons. Those whose eyes are too convex, or who are short-sighted, require the eye-tube to be pushed in, and those whose eyes are somewhat flattened, as old people, require the tube to be drawn out. Indeed there are scarcely two persons whose eyes do not require different adjustments in a slight degree. In some cases I have found that the difference of adjustment for two individuals, in order to produce distinct vision in each, amounted to nearly half an inch. Hence the difficulty of exhibiting the sun, moon, and planets through telescopes, and even terrestrial objects, to a company of persons who are unacquainted with the mode of using or adjusting such instruments--not one half of whom generally see the object distinctly--for, upon the proper adjustment of a telescope to the eye, the accuracy of vision, in all cases, depends; and no one except the individual actually looking through the instrument, can be certain that it is accurately adjusted to his eye, and even the individual himself, from not being accustomed to the view of certain objects, may be uncertain whether or not the adjustment be correct. I have found by experience that when the magnifying powers are high, as 150 or 200, the difference of adjustment required for different eyes is very slight; but when low powers are used, as 20, 30, or 40, the difference of the requisite adjustments is sometimes very considerable, amounting to 1/4 or 1/2 of an inch.

2. _State of the Atmosphere most proper for observing terrestrial and celestial objects._ The atmosphere which is thrown around the globe--while it is essentially requisite to the physical constitution of our world, and the comfort of its inhabitants--is found in many instances a serious obstruction to the accurate performance of telescopes. Sometimes it is obscured by mists and exhalations, sometimes it is thrown into violent undulations by the heat of the sun and the process of evaporation, and even, in certain cases, where there appears a pure unclouded azure, there is an agitation among its particles and the substances incorporated with them, which prevents the telescope from producing distinct vision either of terrestrial or celestial objects. For viewing distant terrestrial objects, especially with high powers, the best time is early in the morning, a little after sun-rise, and, from that period till about 9 o’clock A.M., in summer; and, in the evening about two or three hours before sun-set. From about 10 o’clock A.M. till 4 or 5 in the afternoon, in summer, if the sky be clear and the sun shining, there is generally a considerable undulation in the atmosphere, occasioned by the solar rays and the rapid evaporation, which prevents high powers from being used with distinctness on any telescope, however excellent. The objects at such times, when powers of 50, 70, or 100 are applied, appear to undulate like the waves of the sea, and, notwithstanding every effort to adjust the telescope, they appear confused and indistinct. Even with very moderate magnifying powers this imperfection is perceptible. In such circumstances, I have sometimes used a power of 200 times on distant land objects, with good effect, a little before sun-set, when, in the forenoon of the same day, I could not have applied a power of 50 with any degree of distinctness. On days when the air is clear, and the atmosphere covered with clouds, terrestrial objects may be viewed with considerably high powers. When there has been a long-continued drought, the atmosphere is then in a very unfit state for enjoying distinct vision with high magnifying powers, on account of the quantity of vapours with which the atmosphere is then surcharged, and the undulations they produce. But, after copious showers of rain, especially if accompanied with high winds, the air is purified, and distant objects appear with greater brilliancy and distinctness than at any other seasons. In using telescopes, the objects at which we look should, if possible, be nearly in a direction opposite to that of the sun. When they are viewed nearly in the direction of the sun, their shadows are turned towards us, and they consequently appear dim and obscure. By not attending to this circumstance, some persons, in trying telescopes, have pronounced a good instrument to be imperfect, which, had it been tried on objects properly illuminated, would have been found to be excellent. In our variable northerly climate the atmosphere is not so clear and serene for telescopic observation as in Italy, the South of France, and in many of the countries which lie within the tropics. The undulations of the air, owing to the causes alluded to above, constitute one of the principal reasons why a telescope magnifying above a hundred times can seldom be used with any good effect in viewing terrestrial objects--though I have sometimes used a power of nearly 200 with considerable distinctness, in the stillness of a summer or autumnal evening, when the rays of the declining sun strongly illuminated distant objects.

The atmosphere is likewise frequently a great obstruction to the distinct perception of _celestial_ objects. It is scarcely possible for one who has not been accustomed to astronomical observations, to form a conception of the very great difference there is in the appearance of some of the heavenly bodies in different states of the atmosphere. There are certain conditions of the atmosphere essentially requisite for making accurate observations with powerful telescopes, and it is but seldom, especially in our climate, that all the favourable circumstances concur. The nights must be very clear and serene--the moon absent--no twilight--no haziness--no violent wind--no sudden change of temperature, as from thaw to frost--and no surcharge of the atmosphere with aqueous vapour. I have frequently found that, on the first and second nights after a thaw, when a strong frost had set in, and when the heavens appeared very brilliant, and the stars vivid and sparkling--the planets, when viewed with high powers, appeared remarkably undefined and indistinct; their margins appeared waving and jagged, and the belts of Jupiter, which at other times were remarkably distinct, were so obscured and ill-defined, that they could with difficulty be traced. This is probably owing to the quantity of aqueous vapour, and perhaps icy particles, then floating in the air, and to the undulations thereby produced. When a hard frost has continued a considerable time, this impediment to distinct observation is in a great measure removed. But I have never enjoyed more accurate and distinct views of the heavenly bodies than in fresh serene evenings, when there was no frost and no wind, and only a few fleecy clouds occasionally hovering around. On such evenings, and on such alone, the highest powers may be applied. I have used magnifying powers on such occasions with good effect, which could not have been applied, so as to ensure distinct vision, more frequently than two or three days in the course of a year.

Sir William Herschel has observed, in reference to this point, ‘In beautiful nights, when the outside of our telescopes is dropping with moisture, discharged from the atmosphere, there are now and then favourable _hours_ in which it is hardly possible to put a limit to the magnifying powers. But such valuable opportunities are extremely scarce, and with large instruments it will always be lost labour to observe at other times. In order therefore, to calculate how long a time it must take to sweep the heavens, as far as they are within the reach of my forty-feet telescope, charged with a magnifying power of 1000, I have had recourse to my journals to find how many favourable hours we may annually hope for in this climate. And, under all favourable circumstances, it appears, that a year which will afford ninety, or at most, one hundred _hours_ is to be called very productive.’ ‘In the equator, with my twenty feet telescope, I have swept over zones of two degrees with a power of 157, but an allowance of ten minutes in Polar distance must be made for lapping the sweeps over one another where they join. As the breadth of the zones may be increased towards the poles, the northern hemisphere may be swept in about 40 zones; to these we must add 19 southern zones; then 59 zones which, on account of the sweeps lapping over one another, about 5 minutes of time in right ascension, we must reckon of 25 hours each, will give 1475 hours. And allowing 100 hours per year, we find that with the 20 feet telescope, the heavens may be swept in about 14 years and three quarters. Now the time of sweeping with different magnifying powers will be as the squares of the powers; and putting _p_ and _t_ for the power and time in the 20 feet telescope, and P = 1000 for the power in the 40 feet instrument, we shall have p^2 : t :: P^2 : ^{tP2}/_{p^2} = 59840. Then making the same allowance for 100 hours per year, it appears that it will require not less than 598 years, to look with the 40 feet reflector, charged with the above-mentioned power, only one single moment into each point of space; and even then, so much of the southern hemisphere will remain unexplored, as will take up 213 years more to examine.’[28]

From the above remarks of so eminent an observer, the reader will perceive how difficult it is to explore the heavens with minuteness and accuracy, and with how many disappointments, arising from the state of the atmosphere, the astronomer must lay his account, when employed in planetary or sidereal investigation. Besides the circumstances now stated, it ought to be noticed that a star or a planet is only in a situation for a high magnifying power, about half the time it is above the horizon. The density of the atmosphere, and the quantity of vapours with which it is charged near the horizon, prevent distinct vision of celestial objects with high powers, till they have risen to at least 15 or 20 degrees in altitude, and the highest magnifiers can scarcely be applied with good effect, unless the object is near the meridian, and at a considerable elevation above the horizon. If the moon be viewed a little after her rising, and afterwards when she comes to her highest elevation in autumn, the difference in her appearance and distinctness will be strikingly perceptible. It is impossible to guess whether a night be well adapted for celestial observations, till we actually make the experiment, and instruments are frequently condemned, when tried at improper seasons, when the atmosphere only is in fault. A certain observer remarks,--‘I have never seen the face of Saturn more distinctly than in a night when the air has been so hazy, that with my naked eye, I could hardly discern a star of less than the third magnitude.’ The degree of the transparency of the air is likewise varying almost in the course of every minute, so that even in the course of the same half hour, planets and stars will appear perfectly defined, and the reverse. The vapours moving and undulating the atmosphere, even when the sky appears clear to the naked eye, will in a few instants destroy the distinctness of vision, and in a few seconds more, the object will resume its clear and well-defined aspect.[29]

3. _On the magnifying powers requisite for observing the phenomena of the different planets--comets--double stars, &c._

There are some objects connected with astronomy which cannot be perceived without having recourse to instruments and to powers of great magnitude. But it is a vulgar error to imagine that very large and very expensive telescopes are absolutely necessary for viewing the greater part of the more interesting scenery of the heavens. Most of the phenomena of the planets, comets and double stars and other objects, are visible with instruments of moderate dimensions, so that every one who has a relish for celestial investigations, may, at a comparatively small expense, procure a telescope, for occasional observations, which will show the principal objects and phenomena described in books on astronomy. Many persons have been misled by some occasional remarks which Sir W. Herschel made, in reference to certain very high powers which he sometimes put, by way of experiment, on some of his telescopes, as if these were the powers requisite for viewing the objects to which he refers. For example, it is stated that he once put a power of 6450 times on his 7 feet Newtonian telescope of 6-3/10 inches aperture; but this was only for the purpose of an experiment, and could be of no use whatever when applied to the moon, the planets and most objects in the heavens. Herschel, through the whole course of his writings, mentions his only having used it _twice_, namely on the stars α Lyræ, and γ Leonis, which stars can be seen more distinctly and sharply defined with a power of 420. To produce a power of 6450 on such a telescope, would require a lens of only 1/77th of an inch in focal distance, and it is questioned by some whether Herschel had lenses of so small a size in his possession, or whether it is possible to form them with accuracy.

_Powers requisite for observing the phenomena of the planets._--The planet Mercury requires a considerable magnifying power, in order to perceive its phases with distinctness. I have seldom viewed this planet with a less power than 100 and 150, with which powers its half moon, its gibbous, and its crescent phase, may be distinctly perceived. With a power of 40, 50, or even 60 times, these phases can with difficulty be seen, especially as it is generally at a low altitude, when such observations are made. The phases of _Venus_ are much more easily distinguished, especially the _crescent_ phase, which is seen to the greatest advantage about a month before and after the inferior conjunction. With a power not exceeding 25 or 30 times, this phase, at such periods, may be easily perceived. It requires, however, much higher powers to perceive distinctly the variations of the gibbous phase; and if this planet be not viewed at a considerably high altitude when in a half-moon or gibbous phase, the obscurity and undulations of the atmosphere near the horizon, prevent such phases from being accurately distinguished, even when high powers are applied. Although certain phenomena of the planets may be seen with such low powers as I have now stated, yet, in every instance, the highest magnifying powers, consistent with distinctness, should be preferred, as the eye is not then strained, and the object appears with a greater degree of magnitude and splendour. The planet _Mars_ requires a considerable degree of magnifying power, even when at its nearest distance from the earth, in order to discern its spots and its gibbous phase. I have never obtained a satisfactory view of the spots which mark the surface, and their relative position, with a less power than 130, 160, or 200 times; and even with such powers, persons not much accustomed to look through telescopes, find a difficulty in distinguishing them.

The strongest and most prominent _belts of Jupiter_, may be seen with a power of about 45; which power may be put upon a 20-inch achromatic, or a 1 foot reflector. But a satisfactory view of all the belts, and the relative positions they occupy, cannot be obtained with much lower powers than 80, 100, or 140. The most common positions of these belts are--one dark and well-defined belt to the south of Jupiter’s equator; another of nearly the same description to the north of it, and one about his north and his south polar circles. These polar belts are much more faint, and consequently not so easily distinguished as the equatorial belts. The _moons_ of this planet, in a very clear night, may sometimes be seen with a pocket 1 foot achromatic glass, magnifying about 15 or 16 times. Some people have pretended that they could see some of these satellites with their naked eye; but this is very doubtful, and it is probable that such persons mistook certain fixed stars which happened to be near Jupiter for his satellites. But, in order to have a clear and interesting view of these, powers of at least 80 or 100 times should be used. In order to perceive their immersions into the shadow of Jupiter, and the exact moment of their emersions from it, a telescope not less than a 44 inch achromatic, with a power of 150 should be employed. When these satellites are viewed through large telescopes with high magnifying powers, they appear with well defined disks, like small planets. The planet Jupiter has generally been considered as a good test by which to try telescopes for celestial purposes. When it is near the meridian and at a high altitude, if its general surface, its belts, and its margin appear distinct and well-defined, it forms a strong presumptive evidence that the instrument is a good one.

The planet _Saturn_ forms one of the most interesting objects for telescopic observation. The _ring_ of Saturn may be _seen_ with a power of 45; but it can only be contemplated with advantage when powers of 100, 150, and 200 are applied to a 3 or a 5 feet achromatic. The _belts of Saturn_ are not to be seen distinctly with an achromatic of less than 2-3/4 inches aperture, or a Gregorian reflector of less than 4 inches aperture, nor with a less magnifying power than 100 times. Sir W. Herschel has drawn this planet with five belts across its disk; but it is seldom that above one or two of them can be seen by moderate-sized telescopes and common observers. The _division_ of the double ring, when the planet is in a favorable position for observation, and in a high altitude, may sometimes be perceived with a 44-inch achromatic, with an aperture of 2-3/4 inches, and with powers of 150 or 180, but higher powers and larger instruments are generally requisite to perceive this phenomenon distinctly; and even when a portion of it is seen at the extremities of the _ansæ_, the division cannot, in every case, be traced along the whole of the half-circumference of the ring which is presented to our eye. Mr. Hadley’s engraving of Saturn, in the ‘Philosophical Transactions’ for 1723, though taken with a Newtonian reflector with a power of 228, represents the division of the ring as seen only on the ansæ or extremities of the elliptic figure in which the ring appears. The best period for observing this division is when the ring appears at its utmost width. In this position it was seen in 1840, and it will appear nearly in the same position in 1855. When the ring appears like a very narrow ellipse, a short time previous to its disappearance, the division, or dark space between the rings, cannot be seen by ordinary instruments.

Sir W. Herschel very properly observes, ‘There is not perhaps another object in the heavens that presents us with such a variety of extraordinary phenomena as the planet Saturn; a magnificent globe, encompassed by a stupendous double ring; attended by seven satellites; ornamented with equatorial belts; compressed at the poles; turning upon its axis; mutually eclipsing its ring and satellites, and eclipsed by them; the most distant of the rings also turning upon its axis, and the same taking place with the farthest of the satellites; all the parts of the system of Saturn occasionally reflecting light on each other; the rings and moons illuminating the nights of the Saturnian, the globe and satellites enlightening the dark parts of the ring; and the planet and rings throwing back the sun’s beams upon the moons, when they are deprived of them at the time of their conjunctions.’ This illustrious astronomer states, that with a new 7 feet mirror of extraordinary distinctness he examined this planet, and found that the ring reflects more light than the body, and with a power of 570 the colour of the body becomes yellowish, while that of the ring remains more white. On March 11, 1780, he tried the powers of 222, 332, and 440 successively, and found the light of Saturn less intense than that of the ring; the colour of the body turning, with the high powers, to a kind of yellow white, while that of the ring still remained white.

Most of the _satellites_ of Saturn are difficult to be perceived with ordinary telescopes, excepting the 4th, which may be seen with powers of from 60 to 100 times. It was discovered by Huygens in 1655, by means of a common refracting telescope 12 feet long, which might magnify about 70 times. The next in brightness to this is the 5th satellite, which Cassini discovered in 1671, by means of a 17 feet refractor, which might carry a power of above 80 times. The 3rd was discovered by the same astronomer in 1672, by a longer telescope; and the 1st and 2nd, in 1684, by means of two excellent object-glasses of 100 and 136 feet, which might have magnified from 200 to 230 times. They were afterwards seen by two other glasses of 70 and 90 feet, made by Campani, and sent from Rome to the Royal Observatory at Paris, by the King’s order, after the discovery of the 3rd and 5th satellites. It is asserted, however, that all those 5 satellites were afterwards seen with a telescope of 34 feet, with an aperture of 3-3/10 inches, which would magnify about 120 times. These satellites, on the whole, except the 4th and 5th, are not easily detected. Dr. Derham, who frequently viewed Saturn through Huygens’ glass of 126 feet focal length, declares, in the preface to his ‘Astro-Theology,’ that he could never perceive above 3 of the satellites. Sir W. Herschel observes, that the visibility of these minute and extremely faint objects, depends more on the _penetrating_ than upon the _magnifying_ power of our telescopes; and that with a 10 feet Newtonian, charged with a magnifying power of only 60, he saw all the 5 old satellites; but the 6th and 7th, which were discovered and were easily seen with his 40-feet telescope, and were also visible in his 20-feet instrument, were not discernible in the 7 or the 10-feet telescopes, though all that _magnifying power_ can do may be done as well with the 7-feet as with any larger instrument. Speaking of the 7th satellite, he says, ‘Even in my 40-feet reflector it appears no bigger than a very small lucid point. I see it, however, very well in the 20-feet reflector; to which the exquisite figure of the speculum not a little contributes.’ A late observer asserts, that in 1825, with a 12-feet achromatic, of 7 inches aperture, made by Tulley, with a power of 150, the 7 satellites were easily visible, but not so easily with a power of 200; and that the planet appeared as bright as brilliantly burnished silver, and the division in the ring and a belt were very plainly distinguished, with a power of 200.

The planet _Uranus_, being generally invisible to the naked eye, is seldom an object of attention to common observers. A considerable magnifying power is requisite to make it appear in a planetary form with a well-defined disk. The best periods for detecting it are, when it is near its opposition to the sun, or when it happens to approximate to any of the other planets, or to a well-known fixed star. When none of these circumstances occur, its position requires to be pointed out by an Equatorial Telescope. On the morning of the 25th January, 1841, this planet happened to be in conjunction with Venus, at which time it was only 4 minutes north of that planet. Several days before this conjunction, I made observations on Uranus. On the evening of the 24th, about 8 hours before the conjunction, the two planets appeared in the same field of the telescope, the one exceedingly splendid, and the other more obscure, but distinct and well-defined. Uranus could not be perceived, either with the naked eye, or with an opera glass; but could be distinguished as a very small star by means of a pocket achromatic telescope magnifying about 14 times. It is questionable whether, under the most favourable circumstances, this planet can ever be distinguished by the naked eye. With magnifying powers of 30 and 70, it appeared as a moderately large star with a steady light, but without any sensible disk. With powers of 120, 180, and 250, it presented a round and pretty well-defined disk, but not so luminous and distinct as it would have done in a higher altitude.

The _Double Stars_ require a great _variety_ of powers, in order to distinguish the small stars that accompany the larger. Some of them are distinguished with moderate powers, while others require pretty large instruments, furnished with high magnifying eye-pieces. I shall therefore select only a few as a specimen. The star _Castor_, or α Geminorum, may be easily seen to be double with powers of from 70 to 100. I have sometimes seen these stars, which are nearly equal in size and colour, with a terrestrial power of 44 on a 44-inch achromatic. The appearance of this star with such powers is somewhat similar to that of η Coronæ in a 7 feet achromatic, of 5 inches aperture, with a power of 500. γ Andromedæ may be seen with a moderate power. In a 30-inch achromatic of 2 inches aperture, and a power of 80, it appears like ε Bootis, when seen in a 5-feet achromatic, with a power of 460. This star is said to be visible even in a 1-foot achromatic with a power of 35. ε _Lyræ_, which is a quintuple star, but appears to the naked eye as a single star,--may be seen to be double with a power of from 6 to 12 time. γ _Leonis_ is visible in a 44-inch achromatic, with a power of 180 or 200. _Rigel_ in a 3-1/2-feet achromatic, may be seen with powers varying from 130 to 200. The small star, however, which accompanies Rigel, is sometimes difficult to be perceived, even with such powers. ε _Bootis_ is seldom distinctly defined with an achromatic of less aperture than 3-1/4 inches, or a reflector of less than 5 inches, with a power of at least 250.

These and similar stars are not to be expected to be seen equally well at all times, even when the magnifying and illuminating powers are properly proportioned; as much depends upon the state of the weather, and the pureness of the atmosphere. In order to perceive the closest of the double stars, Sir W. Herschel recommends, that the power of the telescope should be adjusted upon a star known to be single, of nearly the same altitude, magnitude, and colour with the double star which is to be observed, or upon one star above and another below it. Thus, the late Mr. Aubert, the astronomer, could not see the two stars of γ Leonis, when the focus was adjusted upon that star itself; but he soon observed the small star, after he had adjusted the focus upon Regulus. An exact adjustment of the focus of the instrument is indispensably requisite, in order to perceive such minute objects.

In viewing the _Nebulæ_, and the very small and immensely distant fixed stars, which require much light to render them visible, a large aperture of the object-glass or speculum, which admits of a great quantity of light, is of more importance than high magnifying powers. It is light chiefly, accompanied with a moderate magnifying power, that enables us to _penetrate_ into the distant regions of space. Sir W. Herschel, when sweeping the profundities of the Milky way, and the Hand and Club of Orion, used a telescope of the Newtonian form, 20-feet focal length, and 18-7/10 inches diameter, with a power of only 157. On applying this telescope and power to a part of the _Via Lactea_, he found that it completely resolved the whole whitish appearance into stars, which his former telescopes had not light enough to effect; and which smaller instruments with much higher magnifying powers would not have effected. He tells us, that with this power, ‘the glorious multitude of stars,’ in the vicinity of Orion, ‘of all possible sizes, that presented themselves to view, was truly astonishing, and that he had fields which contained 70, 90 and 110 stars, so that a belt of 15 degrees long, and 2 degrees broad, which passed through the field of the telescope in an hour, could not contain less than fifty thousand stars that were large enough to be distinctly numbered.’ In viewing the Milky way, the Nebulæ, and small clusters of stars, such as _Præsepe_ in Cancer, I generally use a power of 55 times, on an achromatic telescope 6 feet 6 inches in focal length, and 4 inches diameter. The eye-piece, which produces this power--which I formed for the purpose--consists of two convex lenses, the one next the eye 3 inches focal length, and 1-2/10 inch diameter, and that next the object 3-1/2 inches focus, and 1-4/10 inch diameter, the deepest convex surfaces being next each other, and their distance 1/4 inch. With this eye-piece a very large and brilliant field of view is obtained; and I find it preferable to any higher powers in viewing the nebulosities, and clusters of stars. In certain spaces of the heavens, it sometimes presents in one field, nearly a hundred stars. It likewise serves to exhibit a very clear and interesting view of the full moon.

In observing _Comets_, a very small power should generally be used, even on large instruments. These bodies possess so small a quantity of light, and they are so frequently enveloped in a veil of dense atmosphere, that _magnifying_ power sometimes renders them more obscure; and therefore the _illuminating_ power of a large telescope, with a small power, is in all cases to be preferred. A comet eye-piece should be constructed with a very large and uniformly distinct field, and should magnify only from 15 to 30 or 40 times, and the lenses of such an eye-tube should be nearly two inches in diameter. The late Rev. F. Wollaston recommended for observing comets, ‘a telescope with an achromatic object-glass of 16 inches focal length, and 2 inches aperture, with a Ramsden’s eye-glass magnifying about 25 times, mounted on a very firm equatorial stand, the field of view taking in 2 degrees of a great circle.’

In viewing the _moon_, various powers may be applied according to circumstances. The best periods of the moon for inspecting the inequalities on its surface, are either when it assumes a crescent or a half-moon phase, or two or three days after the period of half-moon. Several days after full-moon, and particularly about the third quarter, when this orb is waning, and when the shadows of its mountains and vales are thrown in a different direction from what they are when on the increase,--the most prominent and interesting views may be obtained. The most convenient season for obtaining such views is during the autumnal months, when the moon, about the third quarter, sometimes rises as early as 8 o’clock P.M., and may be viewed at a considerably high altitude by ten or eleven. When in the positions now alluded to, and at a high altitude, very high magnifying powers may sometimes be applied with good effect, especially if the atmosphere be clear and serene. I have sometimes applied a power, in such cases, of 350 times, on a 46-inch achromatic, with considerable distinctness; but it is only two or three times in a year, and when the atmosphere is remarkably favourable, that such a power can be used. The autumnal evenings are generally best fitted for such observations. The _full moon_ is an object which is never seen to advantage with high powers, as no shadows or inequalities on its surface can then be perceived. It forms, however, a very beautiful object, when magnifying powers not higher than 40, 50, or 60 times are used. A power of 45 times, if properly constructed, will show the _whole of the moon_ with a margin around it, when the darker and brighter parts of its surface will present a variegated aspect, and appear somewhat like a map to the eye of the observer.

4. _Mode of exhibiting the Solar spots._

The solar spots may be contemplated with advantage by magnifying powers varying from 60 to 180 times; about 90 times is a good medium power, though they may sometimes be distinguished with very low powers, such as those usually adapted to a one-foot telescope, or even by means of a common opera-glass. The common astronomical eye-pieces given along with achromatic telescopes, and the sun-glasses connected with them, are generally ill-adapted for taking a pleasant and comprehensive view of the solar spots. In the higher magnifying powers, the first eye-glass is generally at too great a distance from the eye, and the sun-glass which is screwed over it, removes it to a still greater distance from the point to which the eye is applied, so that not above one third of the field of view can be taken in. This circumstance renders it difficult to point the instrument to any particular small spot on the solar disk which we wish minutely to inspect; and besides, it prevents us from taking a comprehensive view of the _relative positions_ of all the spots that may at any time be traversing the disk. To obviate this inconvenience, the sun-glass would require to be placed so near to the glass next the eye as almost to touch it. But this is sometimes difficult to be attained, and, in high powers, even the thickness of the sun-glass itself is sufficient to prevent the eye from taking in the whole field of view. For preventing the inconveniences to which I now allude, I generally make use of a _terrestrial_ eye-piece of a considerable power, with a large field, the sun-glass is fixed at the end of a short tube which slides on the eye-piece, and permits the coloured glass to approach within a line or two of the lens next the eye, so that the whole field of the telescope is completely secured. The eye-piece alluded to carries a magnifying power of 95 times for a 46-inch telescope, and takes in about three fourths of the surface of the sun, so that the relative positions of all the spots may generally be perceived at one view. Such a power is, in most cases, quite sufficient for ordinary observations; and I have seldom found any good effect to arise from attempting very high powers, when minutely examining the solar spots.

But, the most pleasant mode of viewing the solar spots--especially when we wish to exhibit them to others--is to throw the image of the sun upon a white screen, placed in a room which is considerably darkened. It is difficult, however, when the sun is at a high altitude, to put this method into practice, on account of the great obliquity with which his rays then fall, which prevents a screen from being placed at any considerable distance from the eye-end of the telescope. The following plan, therefore, is that which I uniformly adopt as being both the easiest and the most satisfactory. A telescope is placed in a convenient position, so as to be directed to the sun. This telescope is furnished with a _diagonal eye-piece_, such as that represented, fig. 77, (p. 344.) The window-shutters of the apartment are all closed, excepting a space sufficient to admit the solar rays; and, when the telescope is properly adjusted, a beautiful image of the sun, with all the spots which then happen to diversify his surface, is thrown upon the _ceiling_ of the room. This image may be from 12 to 20, or 30 inches or more in diameter, according to the distance of the ceiling from the diagonal eye-piece. The greater this distance is, the larger the image. If the sun is at a very high altitude, the image will be elliptical; if he be at no great distance from the horizon, the image will appear circular or nearly so; but in either case the spots will be distinctly depicted, provided the focus of the telescope be accurately adjusted. In this exhibition, the apparent motion of the sun, produced by the rotation of the earth, and the passage of thin fleeces of clouds across the solar disk, exhibit a very pleasing appearance.

By this mode of viewing the solar spots we may easily ascertain their diameter and magnitude, at least to a near approximation. We have only to take a scale of inches, and measure the diameter of any well-defined and remarkable spot, and then the diameter of the solar image; and, comparing the one with the other, we can ascertain the number of miles either lineal or square, comprehended in the dimensions of the spot. For example, suppose a spot to measure one half-inch in diameter, and the whole image of the sun 25 inches, the proportion between the diameter of the spot and that of the sun will be as 1 to 50, in other words, the _one fiftieth_ part of the sun’s diameter. Now, this diameter being 880,000 miles, this number, divided by 50, produces a quotient of 17,600 = the number of miles which its diameter measures. Such a spot will therefore contain an area of 243,285,504, or more than two hundred and forty-three millions of square miles, which is 46 millions of miles more than the whole superficies of the terraquous globe. Again, suppose the diameter of a spot measures 3/10 inch, and the solar image 23 inches, the proportion of the diameter of the spot to that of the sun is as 3 to 230 = the number of tenths in 23 inches. The number of miles in the spot’s diameter will therefore be found by the following proportion: 230 : 880,000 :: 3 : 11,478; that is, the diameter of such a spot measures eleven thousand four hundred and seventy-eight miles. Spots of such sizes are not unfrequently seen to transit the solar disk.

By this mode of viewing the image of the sun, his spots may be exhibited to twenty or thirty individuals at once without the least straining or injury to the eyes; and as no separate screen is requisite, and as the ceilings of rooms are generally white, the experiment may be performed in half a minute without any previous preparation, except screwing on and adjusting the eye-piece. The manner of exhibiting the solar spots, in this way, is represented in fig. 82.

5. _On the space-penetrating power of telescopes._--The power of telescopes to penetrate into the profundity of space is the result of the quantity of light they collect and send to the eye in a state fit for vision. This property of telescopes is sometimes designated by the expression _Illuminating Power_.

Sir W. Herschel appears to have been the first who made a distinction between the _magnifying_ power, and the _space-penetrating_ power of a telescope; and there are many examples which prove that such a distinction ought to be made, especially in the case of large instruments. For example, the small star, or speck of light, which accompanies the pole-star, may be seen through a telescope of large aperture, with a smaller magnifying power than with a telescope of a small aperture furnished with a much higher power. If the magnifying power is sufficient to show the small star completely separated from the rays which surround the large one, this is sufficient in one point of view; but in order that this effect may be produced, so as to render the small star perfectly distinguishable, a certain quantity of light must be admitted into the pupil of the eye--which quantity depends upon the area of the object-glass or speculum of the instrument, or, in other words, on the illuminating power. If we compare a telescope of 2-3/4 inches aperture with one of 5 inches aperture, when the magnifying power of each does not exceed 50 times for terrestrial objects, the effect of illuminating power is not so evident; but if we use a power of 100 for day objects, and 180 for the heavenly bodies, the effects of illuminating power is so clearly perceptible, that objects not only appear brighter, and more clearly visible, in the larger telescope, but with the same magnifying power, they also _appear_ larger, particularly when the satellites of Jupiter and small stars are the objects we are viewing.

Sir W. Herschel remarks, that ‘objects are viewed in their greatest perfection, when, in penetrating space, the magnifying power is so low as only to be sufficient to show the object well--and when, in magnifying objects, by way of examining them minutely, the space-penetrating power is no higher than what will suffice for the purpose; for in the use of either power, the injudicious overcharge of the other will prove hurtful to vision.’ When illuminating power is in too high a degree, the eye is offended by the extreme brightness of the object. When it is in too low a degree, the eye is distressed by its endeavours to see what is beyond its reach; and therefore it is desirable, when we wish to give the eye all the assistance possible, to have the illuminating and the magnifying powers in due proportion. What this proportion is, depends, in a certain degree, upon the brightness of the object. In proportion to its brightness or luminosity, the magnifying power may, to a certain extent, be increased. Sir W. Herschel remarks, in reference to α Lyræ, ‘This star, I surmise, has light enough to bear being magnified, at least a hundred thousand times, with no more than six inches of aperture.’ However beautifully perfect any telescopes may appear, and however sharp their _defining_ power, their performance is limited by their illuminating powers--which are as the squares of the diameters of the apertures of the respective instruments. Thus, a telescope whose object-glass is 4 inches diameter will have four times the quantity of light, or illuminating power, possessed by a telescope whose aperture is only 2 inches, or in the proportion of 16 to 4,--the square of 4 being 16, and the square of 2 being 4.

The nature of the _space-penetrating power_, to which we are adverting, and the distinction between it, and magnifying power, may be illustrated from a few examples taken from Sir W. Herschel’s observations.

The first observation which I shall notice refers to the _nebula_ between η and ζ Ophiuchi, discovered by Messier in 1764. The observation was made with a 10 feet reflector, having a magnifying power of 250, and a space-penetrating power of 28.67. His note is dated May 3, 1783. ‘I see several stars in it, and make no doubt a higher power and more light will resolve it all into stars. This seems to me a good nebula for the purpose of establishing the connection between nebulæ and clusters of stars in general.’--‘June 18, 1784. The same nebula viewed with a Newtonian 20 feet reflector; penetrating power 61, and a magnifying power of 157; a very large and a very bright cluster of excessively compressed stars. The stars are but just visible, and are of unequal magnitudes. The large stars are red, the cluster is a miniature of that near Flamstead’s forty-second Comæ Berenices; Right ascension 17^h 6^m 32^s Polar distance 108° 18´´’ In this case, a penetrating power of about 28, with a magnifying power of 250, barely shewed a few stars; when in the second instrument the illuminating power of 60 with the magnifying power of only 157 showed them completely.

Subsequently to the date of the latter observation, the 20 feet Newtonian telescope was converted into an Herschelian instrument, by taking away the small speculum, and giving the large one the proper inclination for obtaining the front view; by which alteration the illuminating power was increased from 61 to 75, and the advantage derived from the alteration was evident in the discovery of the satellites of Uranus by the altered telescope, which before was incompetent in the point of penetration, or illuminating power. ‘March 14, 1798, I viewed the Georgian planet (or Uranus) with a new 25 feet reflector. Its penetrating power is 95.85, and having just before also viewed it with my 20 feet instrument, I found that with an equal magnifying power of 300, the 25 feet telescope had considerably the advantage of the former.’ The aperture of the 20 feet instrument was 18.8 inches, and that of the 25 feet telescope, 24 inches, so that the superior effect of the latter instrument must have been owing to its greater illuminating power. The following observations show the superior power of the 40 feet telescope as compared with the 20 feet.--‘Feb. 24, 1786, I viewed the nebula near Flamstead’s fifth Serpentis, with my 20 feet reflector, magnifying power 157. The most beautiful extremely compressed cluster of small stars; the greatest part of them gathered together into one brilliant nucleus, evidently consisting of stars, surrounded with many detached gathering stars of the same size and colour. R.A. 15^h 7^m 12^s. P.D. 87° 8´´’--‘May 27, 1791, I viewed the same object with my 40 feet telescope, penetrating power 191.69, magnifying power 370. A beautiful cluster of stars. I counted about 200 of them. The middle of it is so compressed, that it is impossible to distinguish the stars.’--‘Nov. 5, 1791, I viewed Saturn with the 20 and 40 feet telescopes. _Twenty feet._ The fifth satellite of Saturn is very small. The first, second, third, fourth and fifth, and the new sixth satellites are in their calculated places. _Forty feet._ I see the new sixth satellite much better with this instrument than with the 20 feet. The fifth is also much larger here than in the 20 feet, in which it was nearly the same size as a small fixed star, but here it is considerably larger than that star.’

These examples, and many others of a similar kind, explain sufficiently the nature and extent of that species of power that one telescope possesses over another, in consequence of its enlarged aperture; but the exact quantity of this power is in some degree uncertain. To ascertain practically the illuminating power of telescopes, we must try them with equal powers on such objects as the following,--the small stars near the pole-star, and near Rigel and ε Bootis--the division in the ring of Saturn--and distant objects in the twilight or towards the evening. These objects are distinctly seen with a 5 feet achromatic of 3-8/10 inches aperture, and an illuminating power of 144, while they are scarcely visible in a 3-1/2 feet with an aperture of 2-3/4 inches, and an illuminating power of 72, supposing the same magnifying power to be applied. The illuminating power of a telescope is best estimated, in regard to land objects, when it is tried on minute objects, and such as are badly lighted up; and the advantage of a telescope with a large aperture will be most obvious, when it is compared with another of inferior size in the close of the evening, when looking at a printed bill composed of letters of various sizes. As darkness comes on, the use of illuminating power becomes more evident. In a 5 feet telescope some small letters will be legible, which are hardly discernible in the 3-1/2 feet, and in the 2-1/2 feet are quite undefinable, though the magnifying powers be equal. Sir W. Herschel informs us, that in the year 1776, when he had erected a telescope of 20 feet focal length of the Newtonian construction, one of its effects by trial was, that when towards evening, on account of darkness, the natural eye could not penetrate far into space, the telescope possessed that power sufficiently to show, by the dial of a distant church steeple, what o’clock it was, notwithstanding the naked eye could no longer see the steeple itself.

In order to convey an idea of the _numbers_ by which the degree of space-penetrating power is expressed, and the general grounds on which they rest, the following statements may be made. The depth to which the naked eye can penetrate into the spaces of the heavens, is considered as extending to the twelfth order of distances--in other words, it can perceive a star at a distance 12 times farther than those luminaries, such as Sirius, Arcturus or Capella, which, from their vivid light, we presume to be nearest to us. It has been stated above, that Herschel calculated his 10 feet telescope to have a space-penetrating power of 28.67, that is, it could enable us to descry a star 28 times farther distant than the naked eye can reach. His 20 feet Newtonian was considered as having a similar power of 61; his 25 feet, nearly 96, and his 40 feet instrument, a power of 191.69. If each of these numbers be multiplied by 12, the product will indicate how much farther these telescopes will penetrate into space than the nearest range of the fixed stars, such as those of the first magnitude. For instance, the penetrating power of the 40 feet reflector being 191.69, this number multiplied by 12, gives a product of 2,300, which shows, that were there a series of two thousand three hundred stars extended in a line beyond Sirius, Capella and similar stars--each star separated from the one beyond it, by a space equal to the distance of Sirius from the earth--they might be all seen through the 40 feet telescope. In short, the penetrating power of telescopes is a circumstance which requires to be particularly attended to in our observations of celestial phenomena, and in many cases, is of more importance than _magnifying_ power. It is the effect produced by illuminating power that renders telescopes, furnished with comparatively small magnifying powers, much more efficient in observing comets and certain nebulæ and clusters of stars, than when high powers are attempted. Every telescope may be so adjusted, as to produce different space-penetrating powers. If we wish to diminish such a power, we have only to contract the object-glass or speculum, by placing circular rims, or apertures of different degrees of breadth, across the mouth of the great tube of the instrument. But we cannot increase this illuminating power beyond a certain extent, which is limited by the diameter of the object-glass. When we wish illuminating power beyond this limit, we must be furnished with an object-glass or speculum of a larger size; and hence, the rapid advance in price of instruments which have large apertures, and consequently high illuminating powers. Mr. Tulley’s 3-1/2 feet achromatics of 2-3/4 inches aperture, sell at £26 5s. When the aperture is 3-1/4 inches, the price is £42. When 3-3/4 inches, £68 5s. The following table contains a statement of the ‘comparative lengths, apertures, illuminating powers, and prices, of Achromatic Refractors, and Gregorian Reflectors,’ according to Dr. Kitchener.