98 



Mr. Samuel Hunter, 



K'l 



Fi£ 2. 



Fig. 1. 



different dispersive powers an achromatic telescope could be 

 formed. 



" This made me," he writes, " take reflexions into consideration, 

 and finding them regular, so that the angle of reflexion of all 

 sorts of rays was equal to their angle of incidence, I understood 

 that by their mediation optick instruments might be brought to 

 any degree of perfection imaginable, provided [1] a reflecting 

 substance which would polish as finely as glass, and [2] reflect as 

 much light as glass transmits, and [3] the art of communicating 

 to it a parabolic figure be also attained." Here Sir Isaac gives us 

 the three requisites to a good reflecting telescope, but it is with 

 the last alone we have to do at present. 



Let us first see why the figure must be parabolic. Suppose 

 the rays A and A', B and B' fall on a segment of a sphere, A and 

 A' being more remote from the axis than B and B', C being the 

 centre of curvature, Sir Isaac found that the angle of inci- 

 dence AAC was equal the angle of reflection DAC. So also the 

 angle BBC^=EBC. Hence parallel rays falling on a true 

 spherical surface come to a focus at different points, accord- 

 ing to their distance from the axis of curvature DC. Now 

 if the curve at A could be flattened a little, as indicated by 

 the dotted line, it is clear that the angle of incidence, AAC, 

 could be made=the angle EAC and all the rays would come to a 

 focus at the same point E. The curve of which that holds good 

 is called a parabola. The difference between these two curves 



