THE SENSES 



545 



Defects of the Eye. — Eyes which possess the power of seeing 

 objects distinctly a few inches from the eye to infinity are known 

 as Emmetropic (Fig. 164 — 1) ; but all eyes do not possess this 

 range of vision, owing to their shape, or, more correctly, to the 

 length of the eyeball. Myopia, or short sight, is due to the 

 eyeball being too long, whereby the picture is formed in front 

 of the retina, and only a confused and blurred image falls on it 

 (Fig. 164- — 3). The writer's observations show that the majority 

 of horses are slightly short-sighted.* Hypermetropia, or long 



Myopia. 



Fig. 164. — Diagram of an Emmetropic, Hypermetropic, and Myopic 



Eye, to illustrate where the Focal Point exists (Kirke). 



In 2 the short eyeball causes the focus to form behind the retina ; in 3 the long 



eyeball causes the rays to come to a focus in front of the retina. 



sight, is due to the eyeball being too short, whereby, though 

 vision may be perfect for distant objects, those near at hand 

 are not distinctly seen, the picture being brought to a focus 

 behind the retina (Fig. 164 — 2). It is obvious that a concave 

 glass which scatters rays is the remedy for myopia, while a 

 convex lens which converges them is the appropriate glass for 

 hypermetropia. 

 Astigmatism is a defect due to irregularities in the curvature 



* ' The Refractive Character of the Eyes of Horses, ' Proceedings of the 

 Royal Society, No. 334. 1894. 



35 



