296 



A Manual of Veterinary Physiology. 



A ray of light passing from a rarer to a denser medium 

 becomes refracted ; it does not matter whether this medium 

 be water, glass, or diamond, a greater or less bending of the 

 ra} 7 occurs. 



A ray of light passing through a convex lens also 

 becomes refracted, but before we can follow exactly how this 

 refraction occurs, it is necessary that we should be ac- 

 quainted with the various parts of a lens. 



A convex lens has two curved surfaces, and a line drawn 

 through the centre of these two surfaces is known as the 



Fi<;.:>7. Fig. 28. 



floures illustrating the action or lenses upon rays 01 

 Light passing through them. 



Fig. 27. — Biconvex lens ; 0, optical centre ; m, m, chief axis ; „. h, 

 chief or principal rays. 



Fig. 28. — Rays falling upon a lens parallel with the principal axis ,/, .-, 

 are so refracted that they are collected on the other side <<( the 

 lens at a point called the principal focus/'.- the distance Erom < >, 

 the optical centre, to /, is called the focal distance of the lens. 

 The converse of this condition is also true, viz., rays which diverge 

 from a focus pass through the lens, being on the other side parallel 

 with the principal axis without coming together (Landois and 

 Stifling). 



principal axis of the lens. The essential idea of a double 

 convex lens, is that it is thicker at the centre than at the 

 edges. 



Situated on the principal axis of a biconvex Lens a: a 

 point in its interior is the optical centre (Fig. ±~ ; any 

 straight, line passing through the optical centre is termed 

 a secondary axis. 



Parallel rays of light passing through a convex lens, are 

 refracted or bent in such a way that they col!,,'- on the 



