398 optics. 



of the pencils fall upon it parallel ; only, the nearer 

 the eye is to the end of the telescope, the larger 

 the scope or area of the field of view. 



Let c d (Plate 16. %. 3.) be a convex glass 

 fixed in a long tube, and have its focus at E. 

 Then a pencil of rays g h i> flowing 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 point,/; whilst the pencil of 

 rays h I ?n, 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 point A and B, will be formed 

 in the points jf 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 c d, and converge to as many inter- 

 mediate points between e and f; and so will form 

 the whole inverted image e E f, of the distinct 

 object. But because this image is small, a concave 

 glass, n o, 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 converging through 

 the concave glass, but converge less after passing 

 through it than before they go on further, as to 

 b and #, 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 picture a b, upon the retina, whereon it 

 is magnified under the angle b F a. 



But this telescope has one inconvenience which 

 renders it unfit for most purposes; which is, that 

 the pencils of rays being made to diverge by 

 passing through the concave glass n o, very few of 

 them can enter the pupil of the eye j and, therefore, 



