120 ELEMENTARY PHYSIOLOGY. XXIII. 



that while the eye is illuminated the image of the candb 

 is not seen, and gently move the candle up and down. 

 In a few seconds the subdued reddish glare caused by 

 the candle-light will be marked by branching dark lines, 

 which will be seen to form an exact image of the retinal 

 vessels as seen with the ophthalmoscope. The dark lines 

 are shadows of the blood vessels; consequently the struc- 

 tures in which the physiological processes which give 

 rise to the sensation of light begin must lie behind the 

 retinal blood vessels. 



A cup-shaped space, in which the blood vessels are 

 absent, may with care be seen; this is the yellow spot. 



Or, b. Turn the eye inwards towards the nose so as to expose as 

 much as possible of the thin sclerotic behind the cornea. Let an 

 assistant with a lens concentrate the rays of a candle or lamp on 

 the sclerotic as far behind the cornea as possible, so that the rays 

 may pass directly through it towards the opposite side of the eye, 

 and gently move the focus to and fro. The same image is still more 

 distinctly seen. The smaller the focus on the sclerotic, the more 

 distinct the image. 



If the movement of the light is stopped, the image soon fades 

 away. 



In the first method the image moves in the same direction as the 

 light when the light is moved from side to side, but in an opposite 

 direction when moved up and down. 



In the second method the movement of the image is in the same 

 direction as that of the light, whether up and down or from side 

 to side. 



13. The yellow spot. a. Maxwell's method. Place 

 a moderately strong, but perfectly transparent solution 

 of chrome alum in a flat-sided glass vessel. Resting the 

 eye for a minute or two, suddenly look through the 

 vessel at a white cloud. A rosy spot or cloud will appear 



