854 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



in 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.) 



I. m m, chief axis : O, optical centre : rays (>i n) passing through this centre are principal rays, and 

 are not refracted. II. Parallel rays are collected at a focus,/, /O being the focal distance. III. Rays 

 diverging from a point, b, on the chief axis, within the focal distance, pass out of the other side of the 

 lens less divergent, but do not come to a focus. IV. Rays from a source of light, 7, beyond the principal 

 focus,/, again converge on the opposite side of the lens. V. Formation of an inverted image by a convex 

 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 concavo-convex ; D, double concave : E, plano-concave ; 

 F, 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 376). 



Spherical aberration may be corrected in two ways : by increasing 



