392

Appendix II.

is done by Means of a double or treble Oh -jetl-Glass in all those Instruments where anObject is viewed by Means of its Imageformed by refracted Light. The Construc-tion of a Telescope for this Purpose withFig 6 3^* two Object-Glasses in Fig. 3, where ABCDis a Plane Concave Lens of what is calledwhite 'Flint Glass, and CDE a Double Con-vex Lens of green or Crown Glass; ' byMeans of the different refractive Power ofthese two Sorts of Glass and their unequalFigures, it comes to pass that, all the Raysof Light incident upon those Glasses fromdistant radiant Objects, will pass throughthem in such a Manner, that whatever Ab-beration is occasioned in the heterogenealRays in Refraction through the first Glassis so far corrected by the second, that thoseRays emerge from it nearly parallel amongthemselves, and are converged to one Focus(F) forming an Image IM, not sensibly com-pounded or coloured, and therefore moreperfect and distinct when viewed by the Eye-Glasses than that can be which is made bya single Object-Glass.

Now, if those two Glasses could abso-lutely prevent the Abberation of Rays, orproduce an Image entirely free from Con-fusion and Colours, then only three Eye-

G 1 asses