Appendix II.

By these Theorems we solve any Problem.relating to the Composition of Glasses forany given Power of Magnifying; for In-stance, suppose the focal Distance x of the■ Lens M N, were i f Inch, and that of the

to find the

large Glass. H I, ^focal Distance y of the Lens U S, that isto be combined with it at the Distance D— 2 Inches, that shall magnify just as muchas the single Lens M N. We have theAnswer by the second Theorem, where1,5; F = 3, and D — 2 ; hence y =■

x

3 X i,5~ M x 2 4>5

i. So the

3—i,5 i,5

focal Distance of the Lens U S must beone Inch. If D — i, then y — 2 ; and ifD = o, then Y = 3.

VI. Of the latter Improvement of Refract-ing Reflecting Telescopes.

B ESIDES the last mentioned Construc-tion of Telescopes,, which is to belooked upon only as a Correction of di-optric Instruments, there is of late anotherImprovement still greater, which tends notonly to lessen the Abberation of Rays inrefracted Light, but even to prevent anysensible Effects arising therefrom; and thisC c 4 is

391