48 



PHOTOMICROGRAPHY 



a way that, although roughly brought to a focus after 

 emerging, they do not actually pass through one single 

 point. Those rays which pass through an annular zone 

 near the circumference of the lens are focussed to a point 

 on the axis nearer the lens than rays passing through a 

 more central zone. The resulting deviation from an 

 exact focus is called Spherical Aberration, and prevents 

 the formation of a sharp image of the point P. 



Chromatic Aberration is due to the difference of re- 

 frangibility of the different coloured rays which make 

 up white light. Fig. 20 represents, in an exaggerated way, 



Fig. 20 



CHROMATIC ABERRATION 



the course of the blue and red rays LBb and LrR passing 

 through the outer zones of a lens. The blue rays form an 

 image of the point P at B surrounded by a ring of light 

 formed by the red rays, and similarly the red rays form 

 an image at R surrounded by a blue ring bb. The other 

 components of white light form images between B and R. 

 The amount of deviation suffered by any ray depends on 

 the material of the lens, on its shape, and on the wave- 

 length of the light in question. 



Figs. 19 and 20 indicate that a single lens does not 

 give a sharp image of a point, and further difficulties 

 arise in photomicrography from the difference in lumi- 

 nosity and actinic power of the rays of different colour. 

 The yellow-green rays are the brightest, and the image 

 they form is that most accurately focussed on the 



