2S4 



THE SPECIAL SENSES. 



the lens. Lastly, if a luminous point is placed at v, Fig. 117. nearer 

 to the lens than the principal focal distance, the cone of strongly 

 divergent rays that falls upon the lens, although refracted, is still 

 divergent after leaving the lens on the other side and consequently 

 is not focused and forms no real image of the point. For every lens 

 there is a point known as the optical center, and for biconvex lenses 

 this point lies within the lens, o. The line joining this center and 

 the principal focus is the principal axis of the lens (o-F, Fig. 117). 

 All other straight lines passing through the optical center are known 

 as secondary a.n s. Rays of light that are coincident with any of these 

 secondary axes suffer no angular deviation in passing through the 

 lens; they emerge parallel to their line of entrance and practically 

 unchanged in direction. Moreover, any luminous point not on the 



Fig. 118. Diagrams to illustrate the formation of an image by a biconvex lens: a, For- 

 mation of the image of a point ; b, formation of the images of a- series of points. 



principal axis will have its image (conjugate focus) formed some- 

 where upon the secondary axis drawn from this point through the 

 optical center. The exact position of the image of such a point 

 can be determined by the following construction (Fig. 118) : Let A 

 represent the luminous point in question. It will throw a cone of 

 rays upon the lens, the limiting rays of which may be represented by 

 A -h and A-c. One of these rays will be parallel, A-p, and will therefore 

 pass through the principal focus, F. If this distance is determined 

 and is indicated properly in the construction, the line A-p may be 

 drawn, as indicated, so as to pass through F after leaving the lens. 

 The point at which the prolongation of this line cuts the secondary 

 axis. A-o, marks the conjugate focus of A and gives the position at 

 which all of the rays will be focused to form the image, a. In 



