OCTOBEE 19, 1900.] 



SCIENCE. 



599 



mirrors, using them as burning mirrors, as 

 they were called. The first application of 

 an eye lens to the image formed by reflec- 

 tion from a concave mirror appears to have 

 been made by Father Zucchi, an Italian 

 Jesuit. His work was published in 1652, 

 though it appears he employed such an in- 

 strument as early as 1616. The priority, 

 however, of describing, if not making, a 

 practical reflecting telescope belongs to 

 Gregory, who, in his ' Optica Promota,' 

 1663, discusses the forms of images of ob- 

 jects produced by mirrors. He was well 

 aware of the failure of all attempts to per- 

 fect telescopes by using lenses of various 

 curvature, and proposed the form of reflect- 

 ing telescope which bears his name. 



Wewton, however, was the first to con- 

 struct a reflecting telescope, and with it he 

 could see Jupiter's satellites, etc. Encour- 

 aged by this he made another of 6J inches 

 focal length, which magnifled thirty-eight 

 times, and this he presented to the Royal 

 Society on the day of his election to the So- 

 ciety in 1671. 



To Ifewton we owe also the idea of em- 

 ploying pitch, used in the working of the 

 surfaces. 



A third form of telescope was invented 

 by Cassegrain in 1672. He substituted a 

 small convex mirror for the concave mirror 

 in Gregory's form, and thus rendered the 

 telescope a little shorter. 



Short also, from 1730-68, displayed un- 

 common ability in the manufacture of re- 

 flecting telescopes, and succeeded in giving 

 true parabolic and elliptic figures to his 

 specula, besides obtaining a high degree of 

 polish upon them. In Short's first tele- 

 scopes the specula were of glass, as sug- 

 gested by Gregory, but it was not until after 

 Liebig's discovery of the process of deposit- 

 ing 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 Lie- 

 big's discovery Steinheil* — and later, in- 

 dependently, Foucaultf — proposed to em- 

 ploy glass for the specula of telescopes, 

 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 develop- 

 ment 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 construction of each. When the tele- 

 scope 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 suggest the paral- 

 lactic 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, imme- 

 diately. The long focus telescopes of which 

 I have spoken were sometimes used with a 

 tube, but more often the object-glass was 

 mounted in 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 ob- 

 ject to be looked at ; and it was so arranged 

 that by means of a cord it could be brought 

 into a fairly correct position. JSTotwith- 

 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 me- 

 chanical mountings were used, with circles 

 for finding positions, much as we have them 

 now. I have already mentioned the rivalry 



* Gaz. Univ. d' Augsburg, March 24, 1856. 

 ■fCompiesSmd., Vol. XLIV., February, 1857. 



