368 J. J. Woodward on the Oxy-caletum Light 
cular spots appeared as the results of the exposures. The spot 
produced by the electric light was intensely black, that by the 
magnesium of a rich middle-tint, while the circle impressed by 
the calcium light was extremely pale. Want of time prevented 
me from continuing these experiments and obtaining as I de- 
. sired numerical values for the relative actinic powers of these 
sources of illumination under definite conditions; this I have 
however regretted the less, as the actual energy of the naked 
powers from the 3th down, the use of much larger condensers 
to concentrate the light and so to shorten the exposure, and in 
the case of the magnesium light, in the use of a cloek-work 
lamp to increase the steadiness of the illumination. Each of 
these points are in my judgment essential to obtain the best 
results. 
I learn from the same letter of Dr. Maddox that he had him- 
self made experiments with the magnesium lamp some time be- 
fore those of Abercrombie and Wilson. He used powers as 
