3$9

Appendix II.

the Addition of a Lens to £hew it moredistinctly, instead of the three Eye-GlaliesWhich you there fee represented in a com-mon Telescope, there ought to be the Ad- ,dition of two more, or five in all, next theEye, to shew the Object to the best Ad-vantage. These, together with the Ob-ject-Glass, make six Glasses in all, in Tele-scopes of the best Sort.

If AGO represent the small Speculumof a Reflecting Telescope, then will O Bbe the Image formed by the large Specu-lum before it, and I, T, will be the twoGlasses contained in the Eye-Piece of aReflecting Telescope. And, indeed, witha little Reflection it will appear, that iftwo Glasses, by breaking the Refractioninto two Parts, is an Advantage, threeGlasses will be more so, and therefore insome Cases might be successfully applied.

Also since the Distinctness of Vision isby a Composition of Glasses promoted,the Aperture of the Glass next the Eye maybe somewhat encreased, and consequentlythe Field of View thereby enlarged, whichis the great Advantage of the Six-GlassTelescope.

We shall now give the analytical Investi-gation of the focal Distance of the threeGlasses; in order to which,

C c 3 Eej;