470 ON PHYSIOLOGICAL LIMITS OF MICROSCOPIC VISION. 



by the microscope, this image must next be transferred through and 

 by means of the eye structures, to the focal plane of the bacillary 

 layer of rods and co7ies situate at the back of the retina. In the 

 next place the retinal image must represent the microscope image 

 by exact quantitative differences of white light, supposing the 

 object to be colourless, or if the object contains colour, by an exact 

 counterfeit of the coloured microscope image. By quantitative 

 differences of white light is meant the gradation from white to less 

 white and so on to black (by absorption of white light), and the 

 coloured image must necessarily reproduce as many colours and as 

 many gradations of each colour as exist in the object. But the 

 healthiest and most normal eye cannot accomplish all this without 

 loss of light or deduction from accuracy of definition. There are 

 defects in the physical transmission of the image arising from 

 spherical aberration and chromatic dispersion of the eye, as well as 

 from material obstruction to the direct passage of rays of light. 

 The cornea, for instance, has a curvature which differs in the vertical 

 as compared with the horizontal direction, and its substance is far 

 from being perfectly translucent. Then the crystalline lens con- 

 tains within its substance six diverging planes formed by the 

 abutment against each other of the ends of the cell fibres of which 

 the lens is composed. These planes, whether actual fissures filled 

 with granular cement, or boundary lines formed by simple end to 

 end abutment of the cell membrane of the fibres, cause a break of 

 continuity and homogeneity of substance, and the consequence of 

 this arrangement is that bright points seen at a distance appear 

 with a halo of rays, the images of the radiating structure of the 

 lens. The general substance of the lens is also, like that of the 

 cornea, sufficiently milky to affect the passage of light through it, 

 and all these effects increase with acfe. As respects the retina itself 

 a far more deteriorating effect than is commonly supposed to be 

 possible actually occurs iu every healthy normal eye. This is 

 described by Max Schultze in the following terms* : — 



* Zeitschrift fiir wissenschaf t : Zoologie, Band 8, p 91. See Strieker's 

 " Human and Comparative Histology," Sydenham Edition, vol. 3, p. 288. 



