Till; KKTINA 



terminations of this kind, it, is important lo have ;i uniform and moder- 

 ate intensity of illumination, because visual discrimination markedly 

 increases with the light until ;i, certain upper li/nil. has been reached. 

 Purkinje's Figures. The fa.ct 1 1ml t In-sensory elements of the ret in:i, 

 are deeply sealed, is :ilso proved by the phenomenon commonly known 

 as "Purkinjc's images. " l II, li:i,s been pointed out, lh;if, the blood- 

 vessels of the retina r:i,mify upon its innersurfa.ee, whereas I he rods 

 and cones constitute its outermost layer. Consequently, it- might be 

 supposed that all light entering the eye must cast a, sha.dovvof I he blood- 

 vessels upon these sensitive elements. Actually, however, a, dis- 

 turbance of this kind is ob via ted by thcfa,et that I he diameter ol even I he 

 largest, retina,! vessel amounts to only one-^ixtli of the thickness ol the 

 retina, while UK- diameter of the pupil equals 

 only about one-lift h of the dista,nc<! t)ct,ween 

 H,i orifice and the fundus. Under ordinary 

 conditions, then-fore, t ho rods and cones are 



the recipients of the penumbra, of the blood- 

 vessels, while their umbra falls upon the inner 

 layers of the retina. Kxpcrimeril ally , how- 

 ever, we can make use of two or three differ- 

 ent means to render them visible by throwing 

 their shadows upon parts of UK; retina not 

 ordinarily exposed by them. 



If the eye is turned inward and is directed 

 upon a dull background while! UK; attendant 

 reflects a beam of light upon the outer sur- 

 face of the sclera directly behind the cornea, 

 an arborescent, image of the blood-vessels of 

 the illuminated part, of the eyeball will be ob- 

 tained (Kig. 'H2J. In this case, blood-vessel 

 li throws a, shadow upon the neighboring re- 

 tina opposite the beam of light A li. Jf the 



latter is then moved one way or another, the image of these vessels is 

 shifted in the mime direction. Naturally, this stimulation at (' is 

 projected into space through the optical axis as apparently having 

 come from />. This method may also be employed to calculate the 

 distance, between the blood-vessels and the sensory elements of the 

 retina, the factors necessary for this calculation being the distance of 

 UK; background from tin; eye, the dimensions of the eyeball, the angle 

 through which the light is moved, and the apparent movement of UK; 

 image upon I he screen. The values obtained in this way vary be- 

 tween 0.17 and ().:{(> mm. Since it has been determined by histological 

 measurements that the; rods and cones lie at a, distance of from 0.2 to 

 0..3 mm. below the blood-vessels, we have every reason to suppose 

 that the ray of light are received by those particular constituents of 

 the retina. 



1 Boitr. zur KonntnisH den SehenH, Prag, 1819. 



l'|.,. II'.'.. 



I i.i.i'HTKATi'; I'UKKINJK'H I'H.- 



I KKK. 



A, HiMirro of lilit; H, 

 hlood-voHHol; C, Imdow 

 Ilirown by it, which Hliimi- 

 l:ii.ii>ri m projected to 1) upon 



1,1m HiTiri'ii. 



