584 MANUAL OF PHYSIOLOGY. 



layers thrown upon the outer layer of the retina, also shows the 

 latter to be the sensitive one. This phenomenon, known as PUR- 

 KINJE'S FIGURES can be demonstrated as follows. Close the left 

 eye in a dark room, with an evenly dull-colored wall, and while 

 you stare fixedly at the wall hold a candle so that the light can 

 reach the fund us of the eye from the side. With a little practice 

 the least motion of the candle will bring out an arborescent 

 figure on the wall, which exactly corresponds to the retinal ves- 

 sels. It may also be seen by arranging a microscope so as to 

 show a bright light on looking into the instrument and either 

 moving it or the head slightly but constantly the shadows of 

 the retinal vessels will be clearly seen as though they were under 

 the instrument. 



4. The yellow spot, where the retina is chiefly made up of the 

 cone layer, is very much the most sensitive part of the retina, 

 and nearer the ora serrata, where the rods and cones are but 

 badly developed, sight is least acute. 



As in the perception of two points of contact, so we find the 

 retina ceases to be able to distinguish the difference between two 

 luminous points, if they be brought to a focus at a distance of 

 less than .002 mm. from one another. This distance nearly corre- 

 sponds to the diameter of the cones, and it is supposed that the 

 rays from two luminous points must come upon two different cones 

 in order to be visible as two distinct objects. The cones are, how- 

 ever, very irregularly distributed over the retina, being packed 

 closely together at the yellow spot, and scattered more and more 

 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 mm., so that this esti- 

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

 yellow spot, and toward 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 

 " 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." 



