xcviii INTRODUCTION TO OPTICS. 



Fig. 38. Fig. 



objects which are now invisible to us from their minuteness ; for could 

 we bring them close to the eye, their image on the retina would be so 

 much magnified as to render them visible. The microscope is con- 

 structed on. this principle. The single microscope (Jig. 39) consists 

 simply of a convex lens, in the focus of which the object is placed, and 

 through which it is viewed. By this means, you are enabled to bring 

 your eye very near the object, for the lens A B, by diminishing the 

 divergency of the rays before they enter the pupil C, makes them fall 

 parallel on the crystalline humour D, by which they are refracted to a 

 focus -m the retina, at R R. The lens magnifies the object merely by 

 allowing us to bring it nearer to the eye ; those lenses, therefore, which 

 have the shortest focus will magnify the object most, because they enable 

 us to bring the object nearest to the eye. On the other hand, a lens that 

 has the shortest focus is most convex ; and its protuberance will prevent 

 the eye from approaching very near to the object. This inconvenience 

 is remedied by making the lens extremely small : it may then be spherical 

 without occupying much space, and thus unite the advantages of a short 

 focus, and of allowing the eye to approach the object. 



A double microscope is a more complicated instrument (Jig. 40), in 

 which you look not directly at the object AB, but at a magnified 

 image of it, ab. In this microscope two lenses are employed: the one, 

 Jj M, is placed so near the object, that the image which it forms is farther 

 from the lens than the object itself is ; the image therefore is larger than the 

 object itself, and it is further magnified by being viewed through another lens, 



Fig. 40. 



N O, which acts on the principle of the single microscope, and is called the 

 eye-glass. The solar microscope is the most wonderful, from its great 

 magnifying power : in this we also view an image formed by a lens, not 

 the object itself. A ray of light is admitted into a darkened room through 

 a small aperture in the window-shutter, and the object A B (fig. 41), which 

 is a small insect, placed before the lens C D, and nearly at its focus : 



