Proceedings of the Photo grapbical Section. 1083 



Burning magnesium, estimated to produce a light one-twelfth as 

 brilliant as the sun, has been the favorite artificial light for the pho- 

 tographer during the last ten years, but its cost, and the expensive 

 nature of the apparatus, for its proper use, are far too great to admit 

 of general adoption. 



Next, we have the well-known oxy-hydrogen or calcium light, 

 which, although of dazzling brilliancy, gives off onl^' about one 

 seventy-third part the actinic force found in the solar rays. 



Recent experiment, however, has shown that, by siibstitnting for 

 the pencil of oxyd of calcium, one of the compressed oxyd of 

 magnesiutn, the power of this light is so greatly increased as to 

 render it suitable for the production of enlarged negatives and 

 prints on paper, such as are* now classed under the head of solar 

 camera work. Sucli a method of illumination presents several advan- 

 tages, among which are small cost and simplicity of application. 

 Most persons, with little experience, would be able to manage it 

 successfull3% 



Doubtless, one of the principal sources of failure in the use of 

 artificial liglit, has been non-actinic color in the lens through which 

 the light passed for condensation or formation of the image in the 

 camera. 



Many pliotographers have noticed that lenses, after having been in 

 nse a long time, allow the passage of less actinic light, and, conse- 

 quently, require a longer exposure of the subject. In some cases so 

 great has this change been that the lens became utterly worthless for 

 ordinary portraiture. 



The experiments of Faraday, Gaffield, and Pelouze, covering a 

 period of forty-five years, have proved that the action of sunlight 

 produces a change in glass by an exposure of only a few hours, and 

 that such change increases as the exposure is continued. 



Therefore we may readily conclude that some change of a like 

 nature is constantly going on in the lenses of our cameras, even 

 when exposed to diffused solar light. 



Many opticians, in selecting glass, consider only its density and 

 homogeneous structure, totally disregarding tint, unless the color is 

 very decided. 



That great variety of color exists in both flint .and crude glass, 

 especially the latter, may very readily be seen by inspecting the 

 stock of any dealer in the various qualities required for optical 

 purposes. 



