310 OPTICS. 



their thoughts upon the means of converging the 

 rays of light without separating them into their 

 component colours. The best method of effecting 

 this has been accomplished by Dollond. 



This invention consists of a double or treble 

 object-glass ; the double object-glass consists of 

 a double concave of white flint, and a double 

 convex of crown-glass. The parts of the lenses, 

 which are of the same side of the centres, may 

 be conceived to act like two prisms, which re- 

 fract contrary ways ; and if the excess of refrac- 

 tion in the crown-glass be such as to destroy the 

 divergency of colour caused by the flint-glass, the 

 incident ray will be refracted without any produc- 

 tion of colour : the same is true of all the incident 

 rays, and, consequently, the image formed in the 

 focus of this compound object-glass will be free 

 from colours j or, in other words, by means of the 

 different refractive power of these two sorts of 

 glass and their unequal figure, it comes to pass that 

 all the rays of light incident upon those glasses 

 from distant radiant objects, will pass through them 

 in such a manner, that whatever aberration is occa- 

 sioned in the heterogeneal rays in refraction 

 through the flrst glass, is so far corrected by the 

 second, that those rays emerge from it nearly pa- 

 rallel among themselves, and are converged to one 

 focus, forming an image not sensibly compounded 

 or coloured, and, therefore, are more perfect and 

 distinct. It will, therefore, admit a much larger 

 aperture, and, of course, a greater magnifying 

 power than the common refracting telescopes pos- 

 sibly can : if the telescope be short, the convexity 

 of the lenses will be considerable ; and, in such 

 cases, it is most convenient to combine three 



