ELEMENTARY PHOTO-MICROGRAPHY. 77 



of different lengths may destroy each other. The 

 longest wave is red, and the shortest violet. Every 

 colour has a different wave length, and the 

 separating power of objectives increases as the 

 length of light wave diminishes : that is, if the 

 wave length be reduced, it is equivalent to increase 

 of aperture. 



A pamphlet on photo-micrography by the 

 Wratten Division of Kodak, Ltd., Kingsway, 

 London, deals with the selection of light filters 

 for coloured objects in an elementary but thorough 

 manner, and shows clearly how to increase or 

 diminish contrast between the structure of the 

 object and its background ; or, if the object be 

 colour-stained, how best to control the colour of 

 the light used for illumination in order to secure 

 good results. 



From this pamphlet, which can be obtained 

 on application from the firm named, the following 

 extracts are taken : 



" When white light is examined in a spectro- 

 scope, the analysis shows a band of colours which 

 appears to consist of three main portions : red, 

 green, and blue- violet. If objects of various 

 colours be examined, it will be found that a light 

 blue object has an absorption band in the red, 

 a purple object in the yellow, a magenta in the 

 green, an orange in the blue-green, and a yellow 



