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A MANUAL OF VETERINARY PHYSIOLOGY 



are those passing through the centre of the lens — viz., those coin- 

 ciding with the principal or secondary axes. The converse of this 

 is also true — viz., divergent rays proceeding from the principal focus 

 of a lens /pass through and are rendered parallel (Fig. 173). 



The distance from O, the optical centre of the lens, to /, its 

 principal focus, is known as the focal length of the lens. If the diver- 

 gent rays, instead of proceeding from the focus of the lens, proceed 

 from a point / beyond the focus, then the rays, on passing through 

 the lens, are not rendered parallel but convergent (as the refractive 



Fig. 174. — Rays of light passing through a convex lens from I at a point beyond 

 the focus f, cross at some point v, and invert the image. 



power is more than sufficient to render them parallel), and they 

 come to a focus again on the other side of the lens at the point v. 

 The distance from the lens at which they come to a focus depends 

 upon the distance of the luminous point from the lens on the opposite 

 side ; thus the nearer the luminous point / to the principal focus /, 

 the farther will the focus on the opposite side recede, and vice versa. 

 The two foci / and v are termed conjugate foci, and they bear a 

 definite relationship. If the rays of light proceed from a point L 



Fig. 175. 



-Rays of light from a point L, between the focus F and the lens, diverge 

 when passing through a convex lens. 



(Fig. 175), which is nearer to the lens than the principal focus F, 

 the lens is unable to refract the rays sufficiently, and they issue 

 from the opposite side divergent (Fig. 175, dd). 



Parallel rays of light passing through a concave lens, instead of 

 being refracted to a focus, are bent and become divergent, so that 

 a concave lens has no real focus ; but if the divergent rays be pro- 

 duced backwards so as to meet on the principal axis of the lens, the 

 point where they meet is called the negative focus of the lens. 



Spherical Aberration. — The rays of light passing through a convex 

 lens are not all equally refracted, those passing through the circum- 

 ference being more bent than those passing near the centre ; the 

 result is that the rays do not all meet in the same point, those 



