THE RADIANT AND ILLUMINATION 43 



also a good source of illumination, but so un- 

 certain in its intensity at different hours of the 

 day during the varying seasons as to render 

 its employment somewhat difficult and unsatis- 

 factory. Since, in common with direct sun- 

 light, its use is confined to daytime, the night 

 worker cannot employ either, but is obliged 

 to look for some artificial source for his light. 



Fortunately these are numerous enough to 

 suit his circumstances or convenience, what- 

 ever they may be. First and most important, 

 in fact rivaling the direct rays of the sun, is 

 the electric arc light, the current for supply- 

 ing which is now so universally available that 

 few are debarred from use of it, should they 

 so elect. Nor is the question of cost a very 

 serious one any longer. A hand-feed lamp 

 very good may be purchased for a very mod- 

 erate sum. The cost of installation is not 

 large and that of maintenance equally incon- 

 siderable. The light is steady, brilliant and 

 highly actinic, permitting exposures approxi- 

 mating in briefness to those possible with sun- 

 light. I would advise every one to whom it 

 may be available to look carefully into this 

 source of illumination when arranging for an 

 outfit. 



The rays of burning magnesium are proba- 



