418 Veterinary-Captain F. Smith. [Aj.y. 



The area of the most acute vision in the horse js probably the 

 " tapetum," and that portion of the tapetum which lies in the optical 

 axis of the eje is the part where all the observations contained in 

 this paper have been made. 



The horse to be examined is brought into an ordinary stable, and 

 any windows likely to be reflected in the cornea are covered up. The 

 observer stands facing the source of daylight, the eye to be examined 

 being in shadow ; standing opposite to the centre of the pupil (and 

 looking at it in such a direction that a line passing through the eye 

 would come out well below the base of the opposite ear), the light is 

 reflected on to the retina at a minimum distance of 4 feet. A brilliant 

 yellow reflex is obtained from the tapetum, by means of which we 

 can readily fix our position relative to the pupil. 



No means of restraint are employed, and though for a minute or 

 two the horse works the eye under examination to and fro, thereby 

 alternately altering the reflex, yet it soon comes to a standstill as he 

 gets used to the reflection from the mirror. 



The observer, by practice, automatically regulates his position, so 

 as always to be opposite the centre of the longitudinal pupil, and to 

 ensure that during any change he may make in his position he does 

 not come nearer to the eye than the prescribed distance, a 4-foot 

 gauge is lying on the floor at his feet, in the direction of the horse's 

 head. 



The reflex is taken in the horizontal and vertical meridians of the 

 eye; it is very seldom that any difficulty is experienced in deter- 

 mining the direction in which the shadow is travelling ; the real 

 difficulty is in determining between two opposite shadows given con- 

 secutively in the same eye and in the same meridian. I think this 

 latter phenomenon is caused by the horse using its ciliary muscle; 

 with patience the true reflex of the resting eye will be obtained. 



Having determined the direction of the shadows in both meridians, 

 a trial lens is now placed in a frame and held in front of the eye 

 under examination. The glass is brought quite close to the cornea, 

 the centre of the lens corresponding to the centre of the cornea ; the 

 observer, standing at a minimum distance of 4 feet, throws the light 

 into the eye and determines the reflex through the lens. 



As we approach the lens which* corrects the ametropia, some diffi- 

 culty may be experienced in determining the direction taken by the 

 shadow ; sometimes it neither passes " with " nor " against " the 

 mirror, but dies away from the circumference, producing an appear- 

 ance resembling a dissolving view. In human practice this charac- 

 teristic reflex, I believe, is considered to be due to astigmatism. 



No part of the above examination is made under atropine ; as 

 previously explained, the pupil as a rule dilates sufficiently for a 

 complete examination of the fundus to be made, and, so far as 



