THE MICROSCOPE. 9 



Moreover, an object of the same actual size appears of 

 different sizes at different distances from the eye, the 

 magnitude appearing less the greater the distance. This 

 depends on the size of the visual angle being inversely as 

 the distance of the object from the eye. Thus, if we 

 look at a line in the position A (Fig. 5), it produces the 



FIG. 5. Visual angle. 



image a a on the retina : in the position B it appears 

 larger ; because its image b b excites a greater retinal area ; 

 in the latter the visual angle is evidently greater than in the 

 former. 



1 9. Theory of the Microscope. The eye can see the 

 same object clearly when it is successively placed at various 

 distances, because of an alteration of the curvature of the 

 crystalline lens, whereby parallel rays from an object at a 

 great distance, or divergent rays from a near object, may be 

 successively focalised accurately upon the retina. The 

 more minute the object, the nearer one brings it to the eye 

 in order to increase the visual angle, and get as large an 

 image as possible. But there- is a limit to this. Within 

 eight or ten inches from a normal eye of an adult, the 

 object cannot be seen without an unpleasant effort to render 

 the crystalline lens sufficiently convex to focalise the diver- 

 gent rays on the retina ; and within two inches or so the 

 eye completely fails to accomplish the task ; the image is 

 therefore ill defined. But if a convex lens (Fig. 1 9) be in- 

 terposed between the near object and the eye, it acts in the 

 same sense as the crystalline lens, and assists it focussing 

 the divergent rays, and thus the object apparently magni- 

 fied can be clearly seen. 



