668 REPORT— 1900. 



reflecting telescopes, and succeeded in giving true paracolic and elliptic figures to 

 his specula, besides obtaining a bigh degree of polisli upon them. In Short's first 

 telescopes the specula were of glas-s, as suggested by Gregory, but it was not until 

 after Liebig's discovery of the process of depositing a film of metallic silver upon 

 a glass surface from a salt in solution that glass specula became almost universal, 

 and thus replaced the metallic ones of earlier times. ' 



Shortly after the announcement of Liebig's discovery Steinheil ' — and later, 

 independently, Foucault - — proposed to employ glass i'or the specula of tele- 

 scopes, and, as is well known, this is done in all the large reflectors of to-day. 



I now propose to deal with the various steps in the development of the 

 telescope, which have resulted in the three forms that I take as examples of the 

 highest development at the present time. These are the Yerkes telescope at 

 Chicago, my own 5-foot reflector, and the telescope recently erected at the Paris 

 Exhibition, dealing not only with the mountings, but with the principles of con- 

 struction of each. When the telescope was first used all could be seen by holding 

 it in the hand. As the magnifying power increased some kind of support would 

 become absolutely necessary, and this would take the form of the altitude and 

 azimuth stand, and the motion of the heavenly bodies would doubtless sug-gest the 

 parallactic or equatorial movement, by which the telescope followed the object 

 by one movement of an axis placed parallel to the pole. This did not come, 

 however, immediately. The long-focus telescopes of which I have spoken were 

 sometimes used with a tube, but more often the object-glass was mounted iu a 

 long cell and suspended from the top of a pole, at the right height to be in a line 

 between the observer and the object to be looked at ; and it was so arranged that 

 by means of a cord it could be brought into a fairly correct position. Notwith- 

 standing the extreme awkwardness of this arrangement most excellent observations 

 were made in the seventeenth century by the users of these telescopes. Then the 

 achromatic telescope was invented and mechanical mountings were used, with 

 circles for finding positions, much as we have them now. I have already mentioned 

 the rivalry between the English and German forms of mountings, and Airy's 

 preference for the English form. The general feeling amongst astronomers has, 

 however, been largely in favour of the German mounting for refractors, due, no 

 doubt, to a great extent, to the enormous advance in engineering skill. We have 

 many examples of this form of mounting. A list of the principal large refracting 

 and reflecting telescopes now existing is given at the end of this Address. All the 

 refractors in this list, with the exception cf the Paris telescope of 50 inches, and 

 the Greenwich telescope of 28 inches, are mounted on the German form. Some 

 of these carry a reflector as well as, for instance, the telescope lately presented 

 to the Greenwich Observatory by Sir Henry Thompson, which, in addition to a 

 26-inch refractor, carries a 30-inch reflector at the other end of the declination 

 axis, such as had been previously used by Sir W^illiam Huggins and Dr. Roberts ; 

 the last, and perhaps the finest, example of the German form being the Yerkes 

 telescope at Chicago. 



The small reflector made by Sir Isaac Newton, probably the first ever made, 

 and now at the Royal Society, is mounted on a ball, gripped by two curved 

 pieces, attached to the body of the telescope, which allows the telescope to be 

 pointed in any direction. We have not much information as to the mounting of 

 early reflectors. Sir AVilliam Ilerschel mounted his 4-foot telescope on a rough 

 but admirably planned open-work mounting, capable of being turned round, and 

 with means to tilt the telescope to any required angle. This form was not very 

 suitable for picking up objects or determining their position, except indirectly ; but 

 for the way it was used by Sir William Herschel it was most admirably adapted: 

 the telescope being elevated to the required angle, it was left in that position, and 

 became practically a transit instrument. All the objects passing through the field 

 of view (which was of considerable extent, as the eyepiece could be moved in 

 declination) were observed, and their places in time and declination noted, so that 



' Gaz. Univ. d'Aiigslvrrj, March 24, 1856. 

 " CcmjJiei lievyd., vol, xliv, February 1857. 



