CONSTRUCTION OF THE MICROSCOPE. 



39 



to do what might have heen accomplished, though not so 

 well, by one; and the third merely effected certain modi- 

 fications in the light before it en- 

 tered the eye. But in the com- 

 pound microscope the two lenses 

 have totally different functions : the 

 first receives the rays from the ob- 

 ject, and bringing them to new foci, 

 forms an image, which the second 

 lens treats as an original object, and 

 magnifies it just as the single mi- 

 croscope magnified the object itself. 

 Fijr. 26 shows the earliest form of 

 the compound microscope, with the 

 magnified image of a fly, as given 

 by Adams, which he describes as 

 consisting of an object-glass, In, a 

 field glass d e, and an eye-glass, fg; 

 the object, b' o, being placed a little 

 further from the lens than its prin- 

 cipal focal distance, the pencil of 

 rays from which converge to a focus, 

 and form an inverted image of the 

 object at p q, which image is viewed 

 by the eye placed at a through 

 the eye-glass fg. The rays remain 

 parallel after passing out until they 

 reach the eye, when they will con- 

 verge by the refractive powers of 

 this organ, and be collected on the 

 retina. But the image differs from 

 the real object in a very essential 

 particular. The light being emitted 

 from the object in every direc- 

 tion, renders it visible to an eye 

 placed in any position ; but the points of the image formed 

 by a lens emitting no more than a small conical body of 

 rays, which it receives from the glass, can be visible only 

 to the eye situate within its range. Thus the pencil of 

 rays emanating from the object at o, unless converged by 

 the field-lens to /, would cross each other, and diverge 



Fig 26. 



