CONDITIONS AFFECTING VISION. 583 



widely apart as one passes to the peripheral parts of the retina. 

 It is only at the yellow spot in fact that the cones, which here 

 are very thin, are so close together as .002 ram., so that this esti- 

 mation of the size of visual areas only could hold good of the 

 yellow spot, and towards the peripheral parts the power of dis- 

 crimination must be much less keen. This is found to be the case, 

 for in ordinary vision everything seen clearly with a sharp out- 

 line must be brought upon the yellow spot. This is spoken of as 



FJG. 230. 



Section of the retina at the yellow spot, showing the great number of cones 

 (a) at this point, and the thinness of the other layer. (Cadiat.) 



tl direct vision." The images falling on the other parts of the 

 retina are but dim and indistinct outlines, and these are spoken 

 of as "indirect vision." 



The stimulus need only be applied fora very short time in order 

 to cause a distinct sensation, for we can distinctly see a single elec- 

 tric spark ; it need only be applied to an extremely small part of 

 the retina, as we can, by direct vision, see a very minute speck of 

 light, and a very feeble ray suffices to stimulate the retina. The 

 amount of stimulation produced depends upon (1) the intensity of 

 the light, i.e., the amount of light received in a given area ; (2) 

 the duration of its application ; 'and (3) the extent of retina to 

 which it is applied ; (4) the part of the retina stimulated ; (5) 



