558 A MANUAL OF VETERINARY PHYSIOLOGY 



two principal foci, and two principal points ; but with the reduced 

 eye, where there is but one surface and two media, the two nodal 

 points become one, and the two principal points one. 



Dioptrics. — In order to be able to calculate the position of the 

 cardinal points of the eye, certain data must be known, such as the 

 refractive index of the media, the radius of curvature of. each re- 

 fracting surface, the distance from the cornea to the lens, and the 

 thickness of the latter. A very slight error in the determination of 

 these may produce a considerable error in calculation, so that all 

 measurements made by us on the frozen eyes of horses are rejected 

 as wanting in accuracy ; but as an illustration of the measurements 

 of the actual and reduced eye, those furnished by Berlin* are here 

 given. This measurement shows the eye to be long-sighted, the 

 retina being in front of the second principal focus. It will be re- 



m- 



Fig. 176. — The Cardinal Points of a Simple Optical System (Foster). 

 OA, Optic axis ; apb, a curved spherical surface ; n, nodal point ; F 2 , principal 

 posterior focus ; F v principal anterior focus ; ef, rays proceeding from F v 

 rendered parallel to the optic axis ; p, the principal point ; the rays md. 

 Op, and m'e, pass through the nodal point n and undergo no refraction ; 

 the rays cd, parallel to the optic axis, are refracted, and meet at F 2 . 



membered (p. 546) that, according to the writer's observations, 

 the majority of horses are slightly short-sighted, in which case the 

 point F„ will fall in front of the retina. 



The simplified or reduced eye (Fig. 178), consisting of one surface 

 and two media, gives for the horse, according to Berlin, the follow- 

 ing values : 



F, the first principal focus, is situated 27 mm. in front of the 

 cornea. 



F„ the second principal focus, is situated 36 mm. behind the cornea. 



K to a, the distance from the nodal point to the retina, 25* 5 mm. 



H to a, the distance from cornea to retina, 34" 5 mm. 



Formation of a Retinal Image.— Rays of light falling on the 

 eye, as from the arrow XOY (Fig. 179), issue as a pencil of rays 

 from every point of the arrow ; the pencil containing the central 

 ray is known as the principal ray. All principal rays aa' pass 



* Zeitschrift fur Vergleichende Augenheilkunde , Heft 1, 1882. 



