VETERINARY OPHTHALMOLOGY. 131 



gained only by patient application. The observer's 

 eye ninst be normal, i.e., neither hyperopic, myopic or 

 astigmatic, and if such conditions do exist they must 

 be corrected by a proper glass. The indirect method, 

 {the inverted image.) To use this, the examiner holds 

 in front of the observed eye a biconvex lens of 2| 

 or 31 inch focus, and does not bring the scope nearer 

 than one foot, and he may draw gradually back 

 until the proper view is obtained, the top of the scope 



touching the eyebrow. This biconvex lens condenses 

 the light which the mirror throws to the eye, and of 

 necessity (light returning in the same direction in 

 which it came) passes through the lens, becoming- 

 inverted and forming an image between the lens and 

 mirror, in the air, and is thus an aerial image. An im- 

 portant aid to diagnosis is the 24 inch lens which 

 accompanies the ophthalmoscope, and which we use in 

 the indirect method, and also for oblique illumina- 

 tion. In olden times, before oblique illumination, 

 the catoptric test was used to detect cataract, 

 etc., in the lens, but where it was most desir- 



