SIMPLE MICROSCOPE. 19 



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 follow the same proportion. 

 To ordinary eyes, however, there is a limit within which no dis- 

 tinct 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 receive, and to bring to a focus, 

 rays which are parallel or but slightly divergent. This limit is 

 variously stated at from 5 to 10 inches : but though there are 

 doubtless many persons whose vision is good at the shorter range, 

 yet the longer is probably the real limit for persons of ordinary 

 vision ; who, though they may see objects much nearer the eye, see 

 little if any more of their details, since what is gained in size is lost 

 in distinctness. Now the utility of a convex lens interposed between 

 a near' object and the eye, consists in its reducing the divergence of 

 the rays forming the several pencils which issue from it ; so that 

 they enter the eye in a state of moderate divergence, as if they 

 had issued from an object beyond the nearest limit of distinct 

 vision ; and a well-defined picture is consequently formed upon the 

 retina. Not only, however, 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 arrived from a larger object situated at a greater 

 distance. The picture formed upon the retina, therefore, by any 

 object (Fig. 12), corresponds in all respects with one which would 



Fig. 12. 



Diagram illustrating the action of the Simple Microscope : a b, object ; 

 A B its magnified image. 



have been made by the same object a b increased in its dimensions 

 to A b, and viewed at the smallest ordinary distance of distinct 

 vision. A ' short-sighted ' person, however, who can see objects 



C2 



