DIOPTRICS OF THE EYE. 



287 



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 dis- 

 tance. 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 theo- 

 retically by a source of light *at an infinite distance in front of the 

 lens, but practically objects not nearer than about twenty feet 

 give rays so little divergent that they may be considered as par- 



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

 tion of parallel rays ; 6., refraction of divergent rays ; c., refraction of divergent rays from 

 a luminous point nearer than the principal focal distance. 



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

 rays 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 not so far as to give practically 

 parallel rays, 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 



