78 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



object-glass, as we have seen, must consist of a combination of 

 lenses ; and the eye-glass, as we shall presently see ( 21), is best 

 constructed by placing two lenses in a certain relative position, 

 forming what is termed an eye-piece. These two kinds of in- 

 strument need to be separately considered in detail. 



17. Simple Microscope. In order to gain a clear notion of the 

 mode in which a single lens serves to "magnify" minute objects, 

 it is necessary to revert to the phenomena of ordinary vision. 

 An eye free from any defect has a considerable power of adjust- 

 ing itself, in such a manner as to gain a distinct view of objects 

 placed at extremely varying distances ; but the image formed 

 upon the retina will of course vary in size with the distance of 

 the object; and the amount of detail perceptible in it will fol- 

 low the same proportion. To ordinary eyes, however, there is 

 a limit within which no distinct image can be formed, on account 

 of the too great divergence of the rays of the different pencils 

 which then enter the eye ; since the eye is usually adapted to re- 

 ceive, and to bring to a focus, rays which are parallel or but 

 slightly divergent. This limit is variously stated at from five to 

 ten inches ; we are inclined to think from our own observations, 

 that the latter estimate is nearest the truth ; that is, although a 

 person with an ordinary vision may see an object much nearer to 

 his eye, he will see little if any more of its details, since what is 

 gained in size will be lost in distinctness. Now the utility of a 

 convex lens interposed between a near object and the eye, con- 

 sists in its reducing the divergence of the rays forming the seve- 

 ral pencils which issue from it ; so that they enter the eye in a 

 state of moderate divergence, as if they had issued from an ob- 

 ject beyond the nearest limit of distinct vision ; and a well-de- 

 fined picture is consequently formed upon the retina. But not 

 only is the course of the several rays in each pencil altered as 

 regards the rest, by this refracting process, but the course of the 

 pencils themselves is changed, so that they enter the eye under 

 an angle corresponding with that at which they would have ar- 

 rived from a larger object situated at a greater distance. The 

 picture formed upon the retina, therefore,"by any object (Fig. 10), 

 F IG> 10. corresponds in all respects 



with one which would have 

 been made by the same ob- 

 ject a b increased in its di- 

 mensions to A B, and view- 

 ed at the smallest ordinary 

 distance of distinct vision. 

 A "short-sighted" person, 

 however, who can see ob- 

 jects distinctly at a distance 

 of two or three inches, has 

 the same power in his eye 



Diagram illustrating the action of the Simple Microscope, along foy reason of its 



greater convexity, as that which the person of ordinary vision 



