NERVE 



151 



normal far point. The near point is approximated to the 



eye (fig. 70, D). 



(3) Astigmatism is a defect due to unequal curvature of 

 one or more of the refracting surfaces in different planes. 

 If the vertical curvature of the cornea is greater than the 

 horizontal when a vertical line is looked at, horizontal lines 

 cannot be sharply focussed at the same time (fig. 71). 



Errors of refraction are common in the horse, astigmatism 

 being present in something like fifty per cent. 



//. Stimulation of the Retina. 

 1. Reaction to Varying Illuminations. 



(1) The Blind Spot. — At the entrance of the optic nerve 

 the retina cannot be stimulated because there are no end- 



B 



Fig. 72. — Methods of demonstrating the Blind Spot : a, by Mariotte's experi- 

 ment ; b, Area subtending the Blind Spot in which objects cannot be seen. 



organs in that situation. The existence of such a blind spot 

 may be demonstrated in man — 



1st. By Mariotte's experiment, which consists in making 

 two marks in a horizontal line on a piece of paper, closing 



