CELESTIAL OBJECTS. PART I. THE INSTRUMENT AND THE OBSERVER. O multiscium et quovis sceptro pretiosius Perspicillum! an, qui te dextra tenet, ille non rex, non dominus constituatur operum Dei? Vere tu Quod supra caput est, magnos cum motibus orbes THE TELESCOPE. ALTHOUGH the professed design of this volume is to provide a list of objects for common telescopes, it may not be out of place to premise a few remarks upon the instruments so designated. By common telescopes' are here intended such as are most frequently met with in private hands; achromatics of various lengths up to 5 or 6 feet, with apertures* up to 3 or 4 inches; or reflectors of somewhat larger diameter, but in consequence of the loss of light in reflection, not greater brightness. The original observations in the following pages * 'Aperture' always means the clear space which receives the light of the object; the diameter of the object-glass in achromatics, or the large speculum in reflectors, exclusive of its setting. + Maskelyne estimated the apertures of reflectors and achromatics of equal brightness as 8 to 5. Dawes gives this value for Gregorians, but like Herschel II. rates Newtonians as 7 to 5. Steinheil has ascribed much more light to achromatics. Arago strangely asserted that none B were for the most part made with such an instrument, an achromatic by the younger Tulley, 5 feet in focal length,* with an aperture of 37 inches, and of fair defining power; smaller instruments of course will do less, especially with faint objects, but are often very perfect and distinct: and even diminutive glasses, if good, are not to be despised; they will shew something never seen without them. I have a little hand telescope, 224 inches long when fully drawn out, with an object-glass of about 14 inches focus, and 1 inch aperture: this, with an astronomical eye-piece, will shew the existence of the solar spots, the mountains in the Moon, Jupiter's satellites, and Saturn's ring. Achromatics of larger dimensions have become much less expensive than formerly, and silvered specula of considerable size are now comparatively common; even for these it is hoped that this treatise, embodying some of the results of the finest instruments, may not be found an inadequate companion as far as it goes. ances. In judging of a telescope, we must not be led by appearInferior articles may be showily got up, and the outside must go for nothing. Nor is the clearness of the glass, or the polish of the mirror, any sign of excellence: these may exist with bad 'figure' (¿.e. irregular curvature), or bad com was lost in them. The silver-on-glass specula, invented by Foucault and Steinheil, and now manufactured in England, take their place between the metal Newtonian and the achromatic, approaching more nearly to the latter, especially when the plane mirror is replaced by a prism (which, however, does not always conduce to critical definition). Buffham assigns equal light to silvered Newtonians of 9, 63, and 43, and achromatics of 8, 53, and 4 inches respectively. * The focal length is measured from the object-glass, or speculum, to the spot where the rays cross and form a picture of the sun or any celestial body. bination of curves, and the inevitable consequence, bad performance. We need not regard bubbles, sand-holes, scratches, in object-glass or speculum; they merely obstruct a very little light. Actual performance is the only adequate test. The image should be neat and well defined with the highest power, and should come in and out of focus sharply; that is, become indistinct by a very slight motion on either side of it. A proper test-object must be chosen; the Moon is too easy; Venus too severe except for first-rate glasses; large stars have too much glare; Jupiter or Saturn are far better; a close double star is best of all for an experienced eye; but for general purposes a moderate sized star will suffice; its image, in focus, with the highest power, should be a very small disc, almost a point, accurately round, without wings,' or rays, or mistiness, or false images, or appendages, except one or two narrow rings of light, regularly circular, and concentric with the image; and in an uniformly dark field; a slight displacement of the focus either way should enlarge the disc into a luminous circle. If this circle is irregular in outline, or much brighter or fainter towards the centre,† or much better defined on one side of the focus than the other, the telescope * เ The real diameter of a star in the telescope would be inconceivably small. The apparent or 'spurious' disc, and rings, result from the undulatory nature of light. They seem, however, to be somewhat affected by atmospheric causes. Herschel II. speaks of nights of extraordinary distinctness, in which the rings are but traces of rings, all their light being absorbed into the discs.' I have entered 1852, March 23, as ‘a very fine night, though the rings and appendages around the brighter stars were rather troublesome;' 1852, April 1, 'an exceedingly fine night at first, with scarcely a trace of rings or appendages.' See also the star 70 Ophiuchi, in the following catalogue. The small mirror in a Newtonian causes a central darkness out of the focus, which should be nearly the same on either side of it. may be serviceable, but is not of high excellence. The chances are many, however, against any given night being fine enough for such a purpose, and a fair judgment may be made by day from the figures on a watch-face, or a minute white circle on a black ground, or a thermometer bulb in the sunshine, placed as far off as is convenient. An achromatic, notwithstanding the derivation of its name, will shew colour under high powers where there is a great contrast of light and darkness. This 'outstanding' or uncorrected colour results from the want of a perfect balance between the optical properties of the two kinds of glass of which the object-glass is constructed: it cannot be remedied, but it ought not to be obtrusive. In the best instruments it forms a fringe of violet, purple, or blue, round luminous objects in focus under high powers, especially Venus in a dark sky. A red or yellow border would be bad; but before condemning an instrument from such a cause, several eye-pieces should be tried, as the fault might lie there, and be easily and cheaply remedied. Reflectors are most pleasantly exempt from this defect; and as now made by With, and mounted by Browning in the Newtonian form, with specula of silvered glass, well deserve, from their cheapness, combined with admirable defining power, to regain much of the preference which has of late years been accorded to achromatics. The horizontal view of objects at all altitudes is extremely pleasant, when once a little experience has been gained in finding and following: the same advantage, however, attends the use of a diagonal eye-piece with the achromatic. The chief disadvantage of reflectors is the greater aperture, and consequently greater atmospheric disturbance, corresponding with the same amount of light. The eye-piece, or ocular, is only a kind of microscope, magnifying the image formed in the focus of the object-glass or speculum. The size of this image being in proportion to its distance from the glass or mirror which forms it, the power of the same eye-piece in different telescopes varies with the focal length. Hence one disadvantage of a short telescope; to get high powers, we must employ minute and deeply-curved lenses, which are much less pleasant in use: with a telescope twice as long, half the depth in the eye-piece produces an equal power. The focal picture, as in the camera, is always inverted, and so in the astronomical eye-piece it remains.* For terrestrial purposes it is erected by two additional lenses; but a loss of light is thus incurred, and as the inversion of celestial objects is unimportant, erecting eye-pieces (always the longest of a set) should never be employed for astronomy; the eye soon becomes accustomed to the inverted picture, and the hand to the reversed motion in following the object. The lateral vision in the Newtonian reflector interposes another difficulty, easily mastered, however, by practice; the true position of the object being always known from the direction of its motion through the field. A multitude of eye-pieces is needless, but three at least are desirable; one with low power and large field, for extended groups of stars, nebulæ, comets, supplying also, if necessary, the place of a 'finder' for deeper magnifiers; a stronger one for general purposes, especially the moon and planets; and a third, as powerful as the telescope will bear, for minuter objects, especially double stars. A greater number of eye-pieces admits, how and * It is erect in the Galilean eye-piece and the Gregorian reflector. But the use of the former is almost confined to opera-glasses, as its field with high powers is exceedingly small; and the latter is an inferior construction, and now little employed. |