858 



PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



posterior portion of the eyeball the image of external objects in a 

 strong light may be seen upon the retina inverted and reduced in size 

 (Figs. 380 and 381). 



The principal surfaces by which rays of light passing through the 

 eye are refracted are the cornea and the anterior and posterior surfaces 

 of the lens. Since rays of light passing from the atmosphere to the 

 cornea pass from a rarer to a denser medium, the rays of light will be 

 bent toward the perpendicular. The degree of refraction at this point 

 will be more marked than in passing from the cornea to the aqueous 

 humor, both of which possess - the same refractive power. On the 

 other hand, the refraction will be still greater in the lens, so that the 

 rays will be still more strongly deflected inward. It has been seen 

 that rays of light passing through the central point of the optical 

 centre of a convex lens do not undergo refraction, and in a double 

 convex lens of equal curvature the optical centre will coincide with the 



Fig. 380.— Formation of an Image in the Eye. (Landois.) 



By following the rays from the object, A E, it may be seen that they are brought to a focus on the retina, 

 where a small inverted image is formed. 



geometrical centre. In such a compound, system of refracting media 

 as in the eye the optical centre is less readily determined. It has been 

 determined that in the eye the optical centre lies, not exactly in the 

 centre of the crystalline lens, but between that point and the posterior 

 surface of the lens ; consequently, the rays passing through this point 

 will practically undergo no refraction. It has been stated that a screen 

 may be so arranged in relation to a convex lens that a distinct image of 

 the object will fall upon it. This relation is attained when the screen is 

 in the exact focus of the lens. If approached to the lens or removed 

 a greater distance from it the image becomes indistinct. So, also, simi- 

 lar blurring or indistinctness is produced when the distance between 

 the object and the lens is altered. 



In looking at a distant object the rays of light may be regarded as 

 parallel, and, as is well known, are focused with the greatest readiness 

 upon the retina. The difference between the action of the eye in 



