David Gill

“ Partly for these reasons the reflecting telescope with metallic mirror has never been a favourite with the professional astronomer, and has found little employment out of England. In England, in the hands of the Herschels, Rosse, Lassell, and De la Rue it has done splendid service, but in all these cases the astronomer and the instrument-maker were one. The silver-on-glass mirror has the enormous advantage that it can be resilvered with little trouble, at small expense, and without danger of changing the figure. ”

“ Thus an object-glass over-corrected for colour can be improved in this respect by increasing the distance between the lenses. It has been suggested that a telescope can be made suitable for both eye observation and photographic purposes if means are provided for increasing the distance between the lenses without risk of deranging the centring when the telescope is to be employed for photography. But the great change that would be necessary in such a case cannot be brought about consistently with preservation of the perfection of the corrections for spherical aberration. ”

“ Hence one of the best methods of determining the magnifying power of a telescope is to measure the diameter of the emergent pencil of rays, after the telescope has been adjusted to focus upon a star, and to divide the diameter of the object-glass by the diameter of the -emergent pencil. If we desire to utilize all the parallel rays which fall upon an object-glass it is necessary that the full pencil of emerging rays should enter the observer's eye. ”

“ There would be no limit to the rigidity which could be given to such a telescope, as great ease of motion would not have to be considered, and we should abolish all complicated anti-friction apparatus for the declination axis, retaining it only for the polar axis to save wear in the teeth of the driving arc. ”

“ In such a telescope of any considerable aperture the image of a bright star at focus is surrounded by a halo of bluish or violet-coloured light,—a defect which is unavoidable in an object-glass composed of a crown and flint lens on account of the irrationality of their spectra (Light, vol. xiv. p. 592) . There seems to be no doubt that different eyes are differently impressed by rays of different wave-length. ”

“ The process of casting and annealing, in the case of the specula of the great Melbourne telescope, was admirably described by Dr Robinson in Phil. Trans., 1869, vol. clix. p. 135. Shaping, polishing, and figuring of specula are accomplished by methods and tools precisely similar to those employed in the construction of lenses. The reflecting surface is first ground to a spherical form, the parabolic figure being given in the final process by regulating the size of the pitch squares and the stroke of the polishing machine. The process of testing is identical with that of an object-glass. ”

“ It is not a little curious that the obvious improvement of transferring the declination circle as well as the declination clamp to the telescope end of the declination axis was so long delayed; we can ascribe the delay only to a desire to retain the decimation circle as part of the counterpoise. We believe that the first important equatorials in which the declination axis was read from the eye end were the 15-inch by Grubb and the 6-inch by Cooke, made for the observatory of Lord Crawford (then Lord Lindsay) at Dun Echt (Aberdeenshire) about 1873. The plan is now almost universally adopted. ”

“ Gregory.James Gregory, in his Optica Promota (1663) , discusses the forms of images of objects produced by lenses and mirrors, and shows that when the surfaces of the lenses or mirrors are portions of spheres the images are curves concave towards the objective, but if the curves of the surfaces are conic sections the spherical aberration is corrected. He was well aware of the failures of all attempts to perfect telescopes by employing lenses of various forms of curvature, and accordingly proposed the form of reflecting telescope which bears his name. ”

“ Were we called upon to design a great refractor we should abolish all such apparatus and provide the observer with a few conveniently placed small handles or keys for electrical connexions, and we should perform all motions of the telescope whatsoever by electromotors. There is no form of energy so convenient for the astronomer. ”

“ The final correction for achromatism is made, if necessary, by departing slightly from a plane in the curvature of the last surface of the flint lens, and the final correction for spherical aberration in the figuring of the surfaces. In a lecture delivered at the Royal Institution on 2d April 1886 Sir Howard Grubb, optician, of Dublin, said:
"A truly spherical curve is the exception, not the rule. ”

“ These silver-on-glass specula are now the rivals of the achromatic telescope, and it is not probable that many telescopes with metal specula will be made in the future. The best speculum metal and the greatest care are no guarantee of freedom from tarnish, and, if such a mirror is much exposed, as it must be in the hands of an active observer, frequent repolishing will be necessary. ”

“ The total deflexion of the light, to produce the magnifying power, is equally divided between the two glasses,—the most favourable condition for diminishing that distortion which is always perceived in looking obliquely through a lens; and the field of view is greatly enlarged in proportion to the size of the eye-lens, being such as would require, to produce the same magnifying power, a single lens of the much greater semi-diameter bd, found by drawing Qb parallel to qB and erecting bd. ”

“ There is occasional tendency to elliptical motion, and the clock is otherwise troublesome. In the Repsolds' driving clock of the 30 -inch Pulkowa refractor the conical pendulum is reversed, being a heavy weight at the top of a vertical steel rod, kept in conical rotation by a pin at its upper end, which enters a slot in a revolving arm. The rod is in fact a spring of such a form as to cause the revolutions to be nearly or perfectly isochronous whatever the angle of the cone of motion; the clock is therefore, within limits, independent of the power applied to it or the force to be overcome. ”

“ The present writer tested the equatorial coudé on double stars at the Paris observatory in 1884, and his last doubts as to the practical value of the instrument were dispelled. He has never seen more perfect optical definition in any of the many telescopes he has employed, and certainly never measured a celestial object in such favourable conditions of physical comfort. The easy position of the observer, the convenient position of the handles for quick and slow motion, and the absolute rigidity of the mounting leave little to be desired. ”

“ The driving sector EF is 5 feet in radius; its circular rim is accurately toothed to fit a square threaded endless screw E, which is turned by the driving clock. A toothed wheel attached to H and acted on by a pinion connected with a hand-wheel affords an easy means of setting the instrument in hour angle, or moving the telescope quickly in right ascension. The telescope is clamped by iron bands to the strong cast-iron cradle, which is cast with and forms one extremity of the declination axis. The counterpoise U is attached to the other extremity. ”

“ For it is easy to understand by the canons above mentioned that the greatest objects may appear exceedingly small, and the contrary, also that the most remote objects may appear just at hand, and the converse; for we can give such figures to transparent bodies, and dispose them in such order with respect to the eye and the objects, that the rays shall be refracted and bent towards any place we please, so that we shall see the object near at hand or at any distance under any angle we please. ”

“ There is only one other plan, that of suspending the observer's chair to the eye end, so that his eye is near the centre of motion of the chair. This is quite practicable for a 36-inch telescope, and one observer, with the necessary guiding keys at hand, could easily work a telescope and dome of the largest dimensions as quickly and with more ease than he could one of 10 or 12 inches aperture. Probably a nervous astronomer would prefer a solid floor to work upon, as in Grubb's proposal ”

“ One cannot but pay a passing tribute of admiration to the men who, with such troublesome tools, achieved such results.
Reflecting telescopes.Until Newton's discovery of the different refrangibility of light of different colours, it was generally supposed that object-glasses of telescopes were subject to no other errors than those which arose from the spherical figure of their surfaces, and the efforts of opticians were chiefly directed to the construction of lenses of other forms of curvature. ”

“ As it would have required a rather severe curvature of the first surface and an unusual convexity of the last to throw the whole convex power into the first lens, using a mere shell of glass to protect the terborate, Professor Stokes thought it more prudent to throw about one-sixth of the whole convex power into the third or crown glass lens, though at the sacrifice of an absolute destruction of secondary dispersion, which by this change from the original design might be expected to be just barely perceptible. ”

“ Glass is lighter, stiffer, less costly, and easier to work than speculum metal. The silvered mirrors have also some advantage in light grasp over those of speculum metal, though, aperture for aperture, the former are inferior to the modern object-glass. ”

“ Mounting of Telescopes.The proper mounting of a telescope is hardly of less importance than its optical perfection. Freedom from tremor, ease and delicacy of movement, facility of directing the instrument to any desired point in the heavens, are the primary qualifications. ”

“ When these weights acquire a certain velocity of rotation the centrifugal force is sufficient to cause the weights to fly out and rub against the inside of the cylinder AA, and their velocity is checked. Instead of a cylinder, the balls may rub against the inside of a hollow cone, and by raising or lowering the axis C the contact of the weights with the cone may be made to take place when the balls have slightly greater or less velocity, and thus the rate of the clock is regulated. A much better arrangement is a modification of Watt's governor, employed by Grubb and Cooke. ”

“ Further, by causing the hour circle, and with it the polar axis, to rotate by clockwork or some other mechanical contrivance at the same angular velocity as the earth on its axis, but in the opposite direction, the telescope will automatically follow a star from rising to setting. ”