DIOPTRIC MECHANISMS OF THE EYEBALL 621 



disc surrounded by a deep red background (Fig. 281). From the middle of the 

 papilla the retinal vessels pass into the eyeball, and they are seen diverging from 

 the papilla to ramify over the rest of the retina. The arteries can be distin- 

 guished from the veins by their brighter red colour as well as by the 

 stronger reflection of light from their surfaces. The yellow spot is very 

 difficult to see, except in atropinised eyes, since it only comes into view 

 when the observed eye is looking straight into the ophthalmoscope. Under 

 these conditions there is a strong ' light reflex,' and the pupil contracts up 

 to a pin-point, unless paralysed by means of atropine. In order to see 

 the blind-spot, or optic disc, the observed eye must be directed inwards ; thus 

 if A is looking at the right eye of B, B must be told to look over A's right 

 shoulder. 





