DIOPTRICS OF THE EYE. 



283 



borne in mind. Parallel rays of light falling upon one surface of the 

 lens are brought to a point or focus (F) behind the other surface 

 (Fig. 117). This focus for parallel rays is the principal focus and the 

 distance of this point from the lens is the principal focal distance. 

 This distance depends upon the curvature of the lens and its 

 refractive power, as measured by the refractive index of the material 

 of which it is composed. Parallel rays are given theoretically by a 

 source of light at an infinite distance in front of the lens. Practi- 

 cally any luminous object not nearer than twenty feet gives parallel 

 rays. On the other hand, if a luminous object is placed at F the rays 



Fig. 117. Diagrams to illustrate the refraction of light by r. convex lens: a., Refrac- 

 tion of parallel rays; b., refraction of divergent rays; c, refraction of divergent rays from 

 a luminous point nearer than the principal focal distance. 



from it that strike upon the lens will emerge from the other surface 

 as parallel rays of light. If a luminous point (/, Fig. 117) is placed 

 in front of such a lens at a distance greater than the principal focal 

 distance, but nearer than about twenty feet, the cone of diverging 

 rays from it that impinges upon the surface of the lens will be brought 

 to a focus (/') further away than the principal focus. Conversely 

 the rays from a luminous point at /' will be brought to a focus at /. 

 These points, / and /', are therefore spoken of as conjugate foci. 

 All luminous points within the limits specified will have their cor- 

 responding conjugate foci, at which their images will be formed by 



