ON THE VARIOUS FORMS OF APPARATUS USED 
FOR POLISHING SPECULA FOR REFLECTING TELE- 
SCOPES. 
BY 
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 v, the violet at v, and the other colours 
at intermediate points. Sir Isaac failedt to discover that with 
different kinds of glass the distance wr, varies (ve. 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. 
t Query—Did this arise from his having used only one kind of glass in his prisms ? 
