CHAPTEK V 



SOURCES OF ILLUMINATION 



THE illuminant that may be adopted is perhaps of primary 

 importance, and at least it can be said that it will to a great 

 extent determine the class of work that may be attempted, as 

 difficulties incidental to the use of the highest powers are con- 

 siderably increased unless a light source of sufficient intensity 

 is used. 



There are at present many sources of illumination available, 

 and these will be dealt with somewhat in the order of their 

 intensity, beginning, that is to say, with the weakest source 

 first. The^following may be taken as those which are at present 

 most generally used : 



Petroleum oil light. 



Welsbach gas. 



Acetylene gas. 



Oxy-hydrogen, or limelight. 



Electric light, of various forms. 



Sunlight. 



Magnesium light has been mentioned also by many workers, 

 but while it may be used for short exposures, it has so many 

 practical difficulties, that on the score both of its inconvenience 

 and of its expense and uncertainty, it hardly deserves much 

 consideration. 



In selecting the illuminant it should be borne in mind that 

 photographic plates at the present day can be obtained of 

 very much greater sensitiveness to any portion of the spectrum 

 than was formerly possible. At the same time, as will be 

 pointed out in considering the selection of suitable photographic 

 plates, it should also be remembered that a slow plate is desirable, 



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