optics. 285 



with ¥ another, and E with F a third, which, with 

 the three singly, make six ; and, lastly, all three 

 combined together, make another, so that, upon 

 the whole, there are seven powers of magnifying 

 with three glasses only. 



The double, or compound microscope, consists of 

 an object-glass c d (Plate 15. fig. 1.), and an eye- 

 glass ef. The small object a b, is placed at a little 

 greater distance from the glass c d, than its prin- 

 cipal focus, so that the pencils of rays flowing from 

 the different points of the object, and passing 

 through the glass, may be made to converge and 

 unite in as many points between g and h t where 

 the image of the object will be formed; which 

 image is viewed by the eye through the eye-glass 

 ef; for the eye-glass being so placed, that the 

 image g k, may be in its focus, and the eye much 

 about the same distance on the other side, the rays 

 of each pencil will be parallel after going out of 

 the eye-glass, as at e and f, till they come to the 

 eye at A-, where they will begin to converge by the 

 refractive powers of the humours ; and, after hav- 

 ing crossed each other in the pupil, and passed 

 through the crystalline and vitreous humours, they 

 will be collected into points on the retina, and 

 form the large inverted image A B thereon. 



The magnifying power of this microscope is as 

 follows : suppose the image g h to be six times the 

 distance of the object a b from the object-glass c d; 

 then will the image be six times the length of the 

 object; but since the image could not be seen dis- 

 tinctly by the bare eye at a less distance than six 

 inches, if it be viewed by an eye-glass e f, of one 

 inch focus, it will thereby be brought six times 

 nearer the eye, and consequently viewed under an 

 angle six times as lars;e as before; so that it will be 



