468 ON PHYSIOLOGICAL LIMITS OF MICEOSCOPIC YISION. 



the retinal image is well defined and its points not blurred by dis- 

 persion circles. And the reason of this will appear on farther 

 consideration of the anatomical distribution of the cones as they lie 

 further from the axial centre of the yellow spot. Just at this 

 centre nothing but closely packed cones is found, and, therefore, 

 acuteness of vision is here greatest, reaching its utmost limit of 

 resolving power. 



" The arrangement of the cones at the yellow spot is surprisingly 

 regular. They are disposed in curved lines which converge towards 

 the centre and produce the appearance]of shagreen or that produced 

 by the engine-turned back of our watches. This arrangement, 

 which on physiological grounds had been predicted by Hensen, is 

 perfectly regular, the cones successively diminishing in diameter 

 from the periphery of the yellow spot to the margin of the 

 forea.""^ 



Outside the centre the distance between the cones gradually 

 increases. At first each cone is surrounded by a ring of rod elements, 

 then by a double and triple row successively, until, as the figures 

 above given show, the distance between their centres increase to 

 gljj, and at the equatorial regions to ~ inch in place of the 

 s^^jj inch at the centre of the yellow spot. From this it follows 

 that a magnifying power of ^^'^ which will form a retinal image 

 of JL inch dimensions from an object having an actual dimension 

 of j^^^ inch will, if this image fall upon part of the retina out- 

 side the yellow spot, just falfil the conditions of separate and distinct 

 appreciation by the retinal cones, and a power of 5-^-^ will not 

 separate the lines or particles by a greater distance than that which 

 separates the cones of the equatorial region. This corresponds with 

 actaal experience in the use of the microscope. For, according to 

 Helmholtz, Abbe, Harting, Dippel, and others, the highest serviceable 

 power for resolving difficult tests does not exceed 500 to 800. And it 

 has been already pointed out that with sach high powers diffraction and 



* Max Schultze, Sydenham Society's Edition of Strieker's Histology, vol» 

 3, page 282. 



