CHAPTEK Y. 

 ILLUMINATION. 



PRACTICALLY we have to consider only five radiants : Sun- 

 light direct, daylight diffused, electric light, oxy-hydrogen 

 limelight, and lamplight from some form of oil lamp. Mag- 

 nesium light we must at once put aside, because, while it has 

 been successfully used for exposures of very brief duration, it 

 is out of the question for prolonged exposures on the 

 scores of inconvenience and expense, and some of our 

 objects will require prolonged exposures to any light how- 

 ever powerful. The light produced by carburetted hydro- 

 gen burned at the orifice of an ordinary " gas-burner " has 

 several qualities which render it useless for our purpose, and 

 no system known to us of burning this gas alone is at all suited 

 to our purpose. Admittedly, however, the incandescence of 

 certain materials impregnated with such refractory substances 

 as zirconium salts, the incandescence being produced by ordin- 

 ary " gas " suitably used, gives some promise of future utility, 

 though as yet we have not been able to utilize any contri- 

 vances such as the " Welsbach " burner, and efforts on our 

 part have not been wanting. 



In past years, before our photographic preparations had at- 

 tained the degree of sensitiveness to light that they have now, 

 duration of exposure was often a very important considera- 

 tion, and it was little wonder that direct sunlight was invari- 

 ably used for certain work, where with even the most power- 

 ful of artificial lights the exposure must have extended to 

 many hours. The .duration of exposure is per se of little con- 

 sequence, but the danger of tremor and change of tempera- 

 ture are much more serious matters. 



The use of direct sunlight involves, in most cases, the use 

 of a heliostat, and in all cases a vast amount of uncertainty. 



