ON PHYSIOLOGICAL LIMITS OF MICROSCOPIC VISION. 469 



illumination are the real difficulties to be overcom e. It is not, therefore, 

 without reason that Helmholtz has so strongly emphasized the part 

 played by the photometric relations of light in the use of high powers, 

 and proved how rapidly the brightness of image falls off as amplifica- 

 tion is increased ; or, that Abbe has insisted upon the importance of 

 perfecting objectives of medium power instead of seeking to obtain 

 enormous amplifications and employing deep eye-pieces. 



In regard to the fact that a highly magnified image must, 

 physiologically, involve the employment of portions of the retina 

 outside the direct axis of vision, it is to be observed that the 

 slightest movement of the eye on its axes will suffice to throw the 

 image upon any required part of the retinal surface round the axial 

 centre, and that this act is constantly being performed in the 

 ordinary use of the eye when attention is directed to such lateral 

 images. 



It is also worthy of notice that the rods which intervene between 

 the cones being equally percipient elements must have their special 

 uses. The thickness of these rods {^ to ^^ inch) is so much 

 less than that of the cones that their function can scarcely be supposed 

 to be that of perceiving form or magnitude. But, as percipient 

 elements greatly outnumbering the cones, an important apprecia- 

 tion of the varying amount of light and shade by which the 

 delineation of form is sustained, and by which the pictorial effect 

 of the microscope is distinguished, may with great probability be 

 attributed to their action. 



In the foregoing observations the physiological conditions of 

 microscope vision have, it is hoped, been made clearer by placing 

 them upon an anatomical basis which, though as yet still far from 

 perfect, indicates at least an actual and natural foundation upon 

 which to build a theory of vision. A very brief reference to the 

 dioptric function of the eye will suffice to shew how greatly 

 microscopic vision is affected by its special excellences and peculiar 

 defects as an optical instrument. 



Supposing every outline, surface, prominence, and depression of 

 an object to be accurately represented in the optical image produced 



