128 



THE MICROSCOPE AND VISUAL ANGLE 



[CH. V 



Now as the visual angle under which any given object is seen 

 depends upon its distance from the eye, and the power of accommo- 

 dation for distance in the eye is limited, if very small objects 

 are to be seen, or the parts of larger objects are to be distinguished as 



FIG. 75. CONSTANT RETINAL IMAGE (R /) AND CONSTANT VISUAL ANGLE 

 WITH VARYING SIZE OF OBJECT AT DIFFERENT DISTANCES. 



R I Retinal image. To keep this of constant size the visual angle must 

 remain constant. 



Object The object varying in size directly as the radius to keep the visual 

 angle and the retinal image constant. 



The radii in this figure are in the proportion of i, 2, 4. 



separate details, there must be some means of enabling the eye to get 

 very close to the object. 



The microscope serves to increase the visual angle under which an 

 object is seen, thus virtually making it possible to get the eye very 

 close to the object and still retain the sharpness of the retinal image. 



