10 



PROPERTIES OP LENSES. 



resulting images, their causes, and the various 

 methods of correcting them. 



When a ray of light passes from one substance 

 through another a part is reflected at the incident 

 surface, a second absorbed in its passage, and a third 

 transmitted. The greater the thickness and number 

 of the transparent media through which the light is 

 transmitted, the greater is the quantity reflected and 

 absorbed. Since Microscopic Objectives are com- 

 posed of several lenses the loss of light is conse- 

 quently very great. 



Spherical Aberration (Fig. 8), shows itself in want 

 FIG. 8. 



of distinctness in an image at the margins when 

 sharp at the centre, and vice versa ; it is caused by the 

 unequal refraction of the rays owing to the spherical 

 shape of the lens, those at the margins coming to a 

 focus before those at the centre. 



Chromatic Aberration (Fig. 9), a ray of white light 



FIG. 9. 



