OF OPTICS. 267 



which is nearest the eye, is called the eye-glass. The LECT. 

 object-glass must be convex., but the eye-glass may be ^^i. 1 !^ 

 either convex or concave : and generally, in looking 

 through a telescope, the eye is in the focus of the eye- 

 glass ; though that is not very material : for the dis- 

 tance of the eye, as to distinct vision, is indifferent, 

 provided the rays of the pencils faH upon it parallel : 

 only, the nearer the eye is to the end of the telescope, 

 the larger is the scope or area of the field of view. 

 Let c d be a convex glass fixed in a long tube, and 



have its focus at E. Then, a pencil of rays gh i, flow- 

 ing from the upper extremity A of the remote object 

 A B, will be so refracted by passing through the glass, 

 as to converge and meet in the pointy'; whilst the 

 pencil of rays k I m, flowing from the lower extremity B, 

 of the same object A B, and passing through the glass, 

 will converge and meet in the point e : and the images of 

 the points A and B, will be formed in the points /'and e. 

 And as all the intermediate points of the object between 

 A and B, send out pencils of rays in the same manner, 

 a sufficient number of these pencils will pass through 

 the object-glass cd, and converge to as many interme- 

 diate points between e andf; and so will form the whole 

 inverted image e Ef, of the distinct object. But because 

 this image is small, a concave glass no is so placed in 

 the end of the tube next the eye, that its virtual focus 

 may be at F, And as the rays of the pencils pass con- 

 verging through the concave glass, but converge less 

 after passing through it than before, they go on further, 

 as to b and a, before they meet ; and the pencils them- 

 selves being made to diverge by passing through the 

 concave glass, they enter the eye, and form the large 

 17. s 



