1 88 OPHTHALMOLOGY FOR VETERINARIANS 



of a building when near to it, but at a distance he can 

 only discern the outHne of the structure. It is the nor- 

 mal visual acuity that we seek to obtain in man when we 

 correct the vision in cases of refractive errors. Fuchs 

 says: ^'We select for the test not one, but two parallel 

 lines, and then determine the greatest distance from the 

 eye at which they can still be perceived as separate ob- 

 jects. From this can readily be calculated the minimum 

 visual angle, which, for a normal eye, amounts to 

 about i'." (Snellen's test-types have been constructed 

 upon the basis of this determination, but for whom this 

 work is intended — the veterinarian — it is unnecessary 

 to go further into this particular subject or to discuss 

 the test-types and their value in the correction of the 

 refractive errors in man.) 



Eyes that are defective range from nearly the normal 

 visual acuity to mere perception of light. These de- 

 fects may be due to errors of refraction or diseases of the 

 retina, chorioid, optic nerve, cornea, or lens. A dis- 

 turbed nutrition of the eyeball may produce a torpor of 

 the retina which causes a reduction in the visual acuity, 

 particularly if the illumination is not perfect. In these 

 cases the vision is greatly reduced, proportionately 

 after dark. 



Accommodation. — Should the power to accommodate 

 vision be paralyzed, the image of an object w^ithin the 

 distance known as infinity — about 20 feet — would be 

 very imperfectly formed upon the retina, because the 



