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 p2 : t :: P2 : tP2/p2 = 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 63/10 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 2¾ 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 2¾ 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 33/10 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.