16 VETERINARY OPHTHALMOLOGY. 



the universal order known as ball-and-socket joint. 

 Luminous impressions upon the retina continue for a 

 short time after cessation of light. If a bright point, 

 like a smouldering or glowing match end, be waved 

 around in a circle, the eye follows it throughout, but if 

 tlie rapidity of its motion be increased it appears drawn 

 out into a curved line, and with higher motion, veiy 

 fast, it becomes a complete ring of light. Sparks from 

 a knife-grinder's wheel become a stream of light. A 

 circular saw with large teeth presents a smooth edge 

 when revolving rapidly, and the spokes of a rapidly- 

 turning wheel assume the appearance of a glimmer- 

 ing disc. 



A brilliant light leaves a longer impression than a 

 dim one. When an electric spark is seen, it has 

 already come to an end, the interval elapsing before its 

 perception by the observer being greater than its 

 actual duration. The momentary closing of the eye- 

 lids in winking is unnoticed, and why ? Because the 

 visual impression of external objects continues unim- 

 paired during the interval occupied by the movements 

 of the lids. The eyes of the horse, remember, are set 

 obliquely. Only in man, apes and some night-birds 

 are the eyes so set as to permit visual lines directly 

 ahead in parallels. 



The bulbus has its poles. An imaginary line from 

 pole to pole is its axis. The equator is at right angles 

 to the axis, so we may have meridians. The visual 



