January 1, 1894.] 



KNOWLEDGE. 



11 



THE GIANT REFRACTING TELESCOPES OF 

 AMERICA. 



By A. C. Ranyard. 



WE live in an age when "records" are almost 

 daily being broken, but it is now almost half 

 a century since the largest telescope yet 

 turned to the heavens was completed by the 

 father of the present Earl of Rosse. He cast 

 his gigantic mirror, six feet in diameter, on the 13th April, 

 1842, and after nearly three years' labour it was mounted 

 at Parsonstown in February, 184;"i. This gi-eat telescope, 

 contained in a tube seven I'eet in diameter, with a focal 

 length of fifty-four feet, still remains the greatest light- 

 grasping instrument which has been turned to the stars ; 

 but its defining powers are very inferior to many smaller 

 instruments which have since been made. 



It was not till 184(3, a year after the completion of the 

 Rosse reflector, that refracting telescopes reached a 

 diameter of fifteen inches. In that year, Merz and Mahler, 

 of Munich, completed two fifteen-inch refractors, one for 

 Harvard College Observatory, and the other for Pulkowa. 

 These comparatively small telescopes quickly showed 

 themselves to be superior in defining power to the great 

 Rosse telescope. With the Harvard telescope Prof. W. C. 

 Bond, in 1848, discovered Hyperion, the eighth satellite 

 of Saturn, a discovery which was almost simultaneously 

 made by Mr. Lassell in England, with a reflector of two 

 feet apertm-e of excellent defining powers. In 18.50 Bond 

 also discovered with the Harvard fifteeu-iuclr refractor the 

 crape ring of Saturn, a discovery which was likewise 

 very nearly simultaneously made in England by Dawes, 

 with a refracting telescope of smaller aperture. 



For about fifteen years the limit of aperture for re- 

 fracting telescopes attained by Merz was not exceeded ; 

 then America took the lead, and in 1860 Alvan Clark, the 

 father of the pi'esent instrument maker, made an eighteen 

 and a half inch refi-actor, with which he discovered the 

 companion to Sirius. For ten years this remained the 

 largest refi'actor in existence ; then England took the lead 

 with a twenty-five inch object glass made by Cooke, of 

 York, for Mr. R. S. Newhall, of Gateshead, in 1870. 

 Three years later, Messrs. Alvan Clark and Son made the 

 twenty-six-inch refractor for the Washington Observatory, 

 with which the satellites of Mars were discovered. Then 

 followed two thirty-inch refractors, one made by the 

 Brothers Henry, of Paris, for the Nice Observatory, and 

 the other by Messrs. Alvan Clark and Son for the Russian 

 Imperial Observatory at Pulkowa ; and, lastly, the tliirty- 

 six-inch refractor made by Alvan Clark and Son for the 

 Lick Observatory. The optical excellence of the great 

 Lick telescope is vouched for by the work done with it by 

 Mr. Burnham on close double stars, and by the discovery 

 made with it by Mr. Barnard of the fifth satellite of 

 .lupiter. 



American astronomers have always shown a preference 

 for refracting telescopes ; with the exception of the late 

 Dr. Henry Draper, who figured and mounted for himself a 

 twenty-eight-inch silver on glass reflector, no American 

 has constructed a reflecting telescope of any considerable 

 .size. Their scientific enthusiasm has restrained them 

 from merely endeavouring to outdo the Earl of Rosse in 

 the construction of instruments of great light-grasping 

 power, and has turned their inventive genius to the difficult 

 problems involved in the evolution and growth of the 

 achromatic telescope. The French still retam the lead 

 as the makers of large discs of optical glass from which 

 the large refractors are made. The disc of flint glass from 

 which the flint lens of the object glass of the Lick telescope 



was made took three years to manufacture. It was 

 repeatedly tested and the defective parts were cut out and 

 replaced by fresh optical glass which was welded to the 

 original mass by pressure at a white heat ; the disc was 

 then slowly allowed to cool, and again tested, and any new 

 defects found were again operated upon. 



The difficulty of manufacturing large homogeneous discs 

 of optical glass is, at present, the chief obstacle in the 

 way of increasing the size of achromatic telescopes. Such 

 large masses of glass are very heavy, and when supported 

 by their edges in a cell their weight causes the lenses to 

 bend ; but a small amount of flexure causes less disturb- 

 ance in the image thrown by a lens than a similar amount 

 of bending of a speculum would cause in the image thrown 

 by a speculum. 



For if a reflecting surface is shifted through a small 

 angle the reflected ray is shifted through double the angle, 

 while a similar change in the position of a refracting 

 surface causes only a very slight shift in the direction of 

 the refracted ray. Thus it happens that though lenses 

 can only be supported by their edges, while specula can 

 be supported at their backs as well as at their edges, the 

 small distortions due to the weight of material cause a 

 greater disturbance in the image thrown by a large 

 reflector than in the image thrown by a large refractor ; 

 otherwise large reflectors would be vastly superior 

 instruments to large refractors, for with reflectors light 

 of all colours is brought to the same focus, while 

 the difficulty of achromatizing a lens increases with its 

 diameter, and the larger a refractor the greater must be 

 the ratio of focal length to aperture in order to obtain the 

 same degree of achromatism. Thus the focal length of the 

 Lick telescope is fifty-seven feet ten inches, to an aperture 

 of thirty-six inches, and the Yerkes telescope will have an 

 aperture of forty inches, and a focal length of nearly 

 sixty-four feet, or a ratio rather less than one to nineteen 

 instead of one to twelve as with small achromatics. 



With every increase in focal length all the mechanical 

 difficulties connected with the mounting and the size of 

 dome and shutter increase as the cube of the focal length, 

 for the weight of all the parts increases as the cube of their 

 lineal dimensions. Consequently every efl'ort is made to 

 keep down the focal length of such large telescopes, so far 

 as it reasonably can be kept down consistently with a 

 sufficient degree of achromatization for defining purposes. 

 An observer who looks through one of these large refracting 

 telescopes for the first time is greatly struck with the 

 amount of outstanding colour around the stars. The 

 violet end of the spectrum is generally left outstanding, 

 and the star appears bathed in a field of violet light, which 

 however, interferes very little with the sharpness of vision. 



Our second plate shows the mounting of the great 

 telescope which Mr. Charles T. Y'erkes is presenting to 

 the University of Chicago. It is dwarfed by the size of 

 the Exhibition building, but if the size of the figures 

 standing in the neighbourhood is compared with the 

 telescope it will be seen that it is much larger than the 

 Lick telescope, which at first sight looks larger. 



The lenses for the Yerkes telescope are already roughly 

 figured, but they are not finished. Mr. Alvan Clark was 

 present at the last meeting of the Astronomical Society, 

 and gave the Fellows an account of the way in which the 

 work is proceeding. I have also received some additional 

 information from Prof. Hale, who is at present in Berhn. 



The flint lens is very white. It is a particularly fine 

 piece of glass and weighs about three hundred pounds. 

 Its thickness is about one and a half inches at the centre 

 and two and three-quarter inches near the circumference. 



The crown lens weighs about two hundred pounds. It 



