REFRJNGiniL/fr of LIGHT. 25 



ed towards a diftant object, the rays will enter it, as to fenfe, 

 perpendicularly, and will therefore fuffer no refraction. If the 

 convex furface of this lens be brought in contact, with a fluid 

 of lefs mean refractive denfity than the glafs, but exceeding it 

 in difperfive power, in that degree which occafions an equal re- 

 fraction of all the rays, all thefe rays will then be converged to 

 the fame point, which are incident at the fame diftance from 

 the axis of the lens. The focal diftance of this compound lens 

 will be greater or lefs in proportion to its radius of convexity, 

 and to the difference of refraction between it and the fluid made 

 ufe of. While the fluid is confined on one fide by the plano- 

 convex lens, let the lens which is brought in contact with it on 

 the oppofite fide, have one of its fides ground convex, and the 

 other concave ; the radii of their fphericities being equal to the 

 focal diftance at which the rays are made to converge, by the 

 refraction which takes place, when light pafTes from the plano- 

 convex lens into the fluid. It is manifeft that the light will 

 now both enter into this compound lens, and emerge from it 

 perpendicularly, and will therefore fuffer ho refraction, except 

 in the confine of the convex fide of the plano-convex and the 

 difperfive fluid, where all the rays are equally refrangible. A 

 compound lens of this kind, is reprefented in the ninth figure, 

 which, after what has been faid, will require no farther expla- 

 nation ', excepting only, that inftead of being fpherical, it is re- 

 prefented with that curvature which converges homogeneal rays, 

 incident, at all diftances from the axis, to the fame point. 

 If the required curvature could be given to lenfes with fuffi- 

 cient accuracy, this figure feems to reprefent as perfect a con- 

 ftructioh of the object-glafs of a telefcope as can be defired. 

 But there is reafon to think that a fpherical figure may be com- 

 municated, not only much eafier, but with greater accuracy than 

 a fpheroidal or hyperboloidal, which would be required ; and 

 even if this difficulty could be got over, there would ftill re- 

 main a fundamental fault in the theory. Before relating the 

 Vol. III. D obfervations 



