ON THE VARIOUS FORMS OF APPARATUS USED 

 FOR POLISHING SPECULA FOR REFLECTING TELE- 

 SCOPES. 



BT 



SAMUEL HUNTER, f.r.a.s. 

 [Read 17th December, 1877.] 



We owe the reflecting telescope to Sir Isaac Newton,* and the 

 idea was presented to his mind through a remarkable mistake he 

 made in believing that refracting telescopes as then made were 

 incapable of further improvement. In a paper presented to the 

 Royal Society in February, 1672, he writes : — " In the beginning 

 of 1666 (at which time I applied myself to the grinding of optical 

 glasses of other figures than spherical) I procured me a triangular 

 prisme to try therewith the celebrated phenomenon of colours," 

 which "was at first a very pleasing divertissement." Starting 

 from this "divertissement" he found that the colours observed in 

 the telescopes, as then constructed with object-glasses consisting of 

 a single lens, were due to the nature of light. He writes — " When 

 I understood this I left off my aforesaid glass works, for I saw that 

 the perfection of telescopes was hitherto limited — not so much for 

 want of glasses truly figured — as because that light itself is a 

 heterogeneous mixture of differently refrangible rays; so that 

 were a glass so exactly figured as to collect any one sort of rays 

 into one point, it could not collect those also into the same point 

 which, having the same incidence upon the same medium, are apt 

 to suffer a different refraction." That is, he discovered that rays of 

 light falling on a single lens at AA' become decomposed into 

 the colours of the spectrum on leaving it at B B', the red rays 

 coming to a focus at r, the violet at v, and the other colours 

 at intermediate points. Sir Isaac failedt to discover that with 

 different kinds of glass the distance vr, varies (i.e. their dis- 

 persion) and hence did not see that by combining glasses of 



* Gregory described his form of reflecting telescope in 1663, and had one constructed in 

 1664, but failed in obtaining a satisfactory result. 



f Query — Did this arise from his having used only one kind of glass in his prisms ? 



