PERSPECTIVE GLASSES 17 



NON-ACHROMATIC TELESCOPES 



88. Hnch Ivory mounted i-draw Galilean or Perspective Glass. 



Orrery Coll. 26. 



Outer body ornamented with gold tooling, 4 inches long when 

 closed, 4-1 inches when extended. Draw-tube covered with green vellum. 

 Object glass, 4^ inches focus, and eyepiece are mounted in ivory cells 

 with screw caps. 



We place this example of the ' least kind of Perspective Glasses ' 

 first because it represents the telescope with the concave eye lens, the 

 invention of Galileo in 1609, so famous for the great discoveries he 

 immediately made with it and now so familiar in the form of the 

 common opera-glass. Galileo's best telescope is said to have been 



3 feet long. Within a century of its invention Galilean telescopes 

 were being made in considerable numbers in Paris. 



PERSPECTIVE GLASSES 



In the eighteenth century the tubes of Perspective glasses were of 

 several sorts and made of various materials. Some are made of stiff 

 paper glued together, covered with parchment or shagreen or leather ; 

 some consist of single tubes, made of light dry wood ; others of several 

 lesser tubes, sliding one within another. The glasses are fixed in the 

 tubes in wooden cells, so as to be easily taken out and put in again, by 

 means of screws, in order that dust or moisture may be readily wiped 

 off from them. At the end of every one of the inward tubes is fitted 

 a wooden ring ( = diaphragm) to hinder the lateral spurious rays from 

 coming to the eye, which is found by experience to be more use than 

 could be thought ; these rings are generally furnished with female screws 

 in those places whereat the glasses are seated. The Perspectives of 3 or 



4 feet, which have but one wooden tube, are most handy, especially, if 

 at each end, instead of covers that screw and unscrew, they have thin 

 sliding pieces of brass for such. 1 



In England Kepler telescopes were made for or by Sir Paul Neile 

 and Messrs. Reeves and Cox. During the latter end of the iyth 

 century Wm. Ball of Mamhead, Devonshire, using an instrument of 

 38 feet focus observed the double nature of Saturn's ring. 



But telescopes of extreme focal length did not find favour in 

 England. Our Savilian Professor Carswell informed Uffenbach in Oct. 

 1710 that ' when H alley was with the famous Hevelius, he discovered 

 that they could not see or observe anything with his telescope of 



1 Stone's 1758 supplement to Bion. Math. Instr., p. 283. 



