CHAPTER IV 



LAMPS AND ILLUMINANTS 



Critical Light. This subject is discussed in Chapter 

 VIII, but it is necessary before describing the different 

 illuminants available for photomicrography to state 

 that the term " Critical " is used in reference to the 

 most delicate microscopical work necessitating the 

 employment of special methods of illumination " Critical 

 Illumination," and that the conditions for the best 

 performance of the objective, and the formation of 

 the " Critical Image," are attained when an image of the 

 radiant is projected across the field of view by means of 

 a substage condenser whose back lens is filled with light ; 

 the image of the radiant being focussed simultaneously 

 with that of the object. 



Suitability of various Radiants for Photomicrography. 

 The great difficulty connected with illumination in 

 photomicrography is to get critical light, and at the 

 same time fill the field of the objective evenly and with 

 sufficient intensity. Several methods, depending on the 

 magnification of the image of the radiant until it 

 covers the field of view, are described in Chapter VIII, 

 and they demand that the intensity of light should 

 be uniform all over the radiant surface. Many of the 

 difficulties encountered in lens making would be sur- 

 mounted and the errors of lenses more easily corrected 

 if a point source of light could be utilised. This is, 

 however, unattainable in practice, and the nearest 

 approach to it must be sought ; a circular radiant 



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