582 MANUAL OF PHYSIOLOGY. 



small letter "a" and moving the page to or from the eye very 

 slowly; a distance may thus be reached when the large letter "A" 

 is quite lost. On approaching the page the eye still fixed on 

 "a" " A" reappears from the inner side ; on withdrawing the 

 page it comes into view from the outer side. This blind spot 

 is not noticed in ordinary vision as we have Jiabitually over- 

 come the deficiency since infancy by our judgments being de- 

 rived from two eyes. By rapid movements one eye hides the 

 deficiency, as seen when attempting the experiment just described. 



3. The fact that when the eye is illuminated in a peculiar way 

 we can see the shadow of the bloodvessels which lie in the inner 

 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 fundusof 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 instru- 

 ment. 



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 



