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 3½-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 3¼ 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 187/10 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 12/10 inch diameter, and that next the object 3½ inches focus, and 14/10 inch diameter, the deepest convex surfaces being next each other, and their distance ¼ 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.