854 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



% ordinary lenses the amount of refraction at the centre and at the 

 circumference is not equal. Rays passing through the optical centre, as 

 already stated, pass directly without refraction, while those which pass 

 near the centre are less refracted than those which pass near the circum- 



Fig. 374— action of a Convex Lens on Light. (Landois.) 



t m m chief axis- O ontical centre; rays (n 11) poBEing through this centre are principal rays, and 

 »™ not re? acted II ParaUei Trays are collected at a focus,/, /O being the focal distance. HI. Rays 



lens. 



ference ; consequently, the amount of refraction increases as the circum- 

 ference of the lens is approached. This is known as spherical aberration. 

 If a screen be placed in the focus of the rays passing near the centre of 



Fig. 375.— Different Kinds of Lenses. (Ganot.) 



A double convex ; B, plano-convex ; C, converging coucavo-convex ; D. double concave ; E, plano-concare ; 

 ' p, diverging concavo-convex ; C and F are also called meniscus lenses. 



the lens the resulting image will be bright in its central portion, and will 

 have surrounding it a halo which becomes fainter and fainter as we pass 

 from the centre to the circumference (Fig 316). 



Spherical aberration may be corrected in two ways : by increasing 



