xxvii.] VISION. 283 



looking steadfastly with one eye towards a wall 1 ; 

 hold the candle to the side of that eye so that 

 while the eye is illuminated the image of the 

 candle 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 structures in which the physio- 

 logical 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, 



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. 



1 A light-coloured wall or \vhite blind is the best. A wall, the 

 paper of which has any very marked pattern, should be avoided. 



