Of Optics. 141

In a refraEling ielefcope , the glass which is nearest-the object inTh trefratiingviewing it, is calied the objeSl-glafs; and that which is nearest the eye, tele f co t e 'is calied the eye-glafi. The object-glass must be convex, but the eye-glass may be either convex or concave: and generally, in lookingthrough a telescope, the eye is in the socus of the eye-glase; thoughthat is not very material: for the distance of the eye, as to distinctvifion, is indifferent, provided the rays of the pencils fall upon it pa-rallel : only, the nearer the eye is to the end of the telescope, thelarger is the scope or area of the field of view.

Let cd be a convex-glase fixed in a long tube, and have its focus Fig.4.at E. Then, a pencil of rays g h i, flowing from the upper extre-mity A of the remote object A B, will be so refracted by paffingthrough the glass, as to converge and meet in the points'; whilst thepencil of rays k lm, flowing from the lower extremi ty B, of the fameobject A B, and paffing through the glass, will converge and meet inthe point e : and the images of the points A and B will be formest inthe points f and e. And as all the intermediate points of the object,between A and B, send out pencils of rays in the fame manner, asufficient number of thefe pencils will pafs through the object-glass cd,and converge to as many intermediate points between e and f-, and sowill form the whole inverted image e Es, of the distinct object. Butbecause this image is fmall, a concave glass n 0 is so placed in theend of the tube next the eye, that its virtual focus may be at F. And asthe pencils of rays pafs converging through the concave glass, but con-verge less after paffing through it than before, they go on further, astob and a, before they meet; and the pencils themselves being madeto diverge by paffing through the concave glase, they enter the eye,and form the large picture a b upon the retina, whereon it is magnifiedunder the angle b F a.

But this telescope has one inconveniency which renders it unfit formost purposes, which is, that the pencils of rays being made to di-verge by paffing through the concave glase n 0, very few of them canenter the pupil of the eye; and therefore the field of view is but veryfmall, as is evident by the figure. For none of the pencils which floweither from the top or bottom of the object A B can enter the pupilof the eye at C, but are all stopt by falling upon the iris above and be-low the pupil: and therefore, only the middle part of the objectcan be feen when the telescope lies directly towards it, by means ofthose rays which proceed from the middle of the object. So that tofee the whole of it, the telescope must be moved upwards and down-wards, unlese the object be very remote; and then it is never feendistinctly. This