MOM Aas ii | as applied to Photo-micrography. 65 
This circumstance, however, renders the Calcium light inferior to 
the sun and the Electric lamp, in the resolution of the Nobert’s 
plate and certain lined test-objects. 
I did not find the time of exposure differed materially from 
what I had given in making photographs of the same objects with 
the Magnesium lamp, and the pictures produced were not inferior 
. to these in quality. This arose from the fact that the greater 
steadiness of the Calcium light permitted the use of condensers 
which concentrated the light to a greater degree than I had found 
advantageous with the Magnesium lamp, and not from equality in 
the actinic power of the two sources of illumination. I have 
recently made some experiments with the view of obtaining positive 
information with regard to the comparative actinic energy of the 
Electric, Magnesium, and Calcium lamps which I employ. For 
this purpose, all condensers being removed, the divergent pencil 
proceeding from each lamp in turn was permitted to fall, for the 
space of five seconds, on an exposed circular portion of a sensitive 
plate thirty feet distant. The whole operation was completed in 
less than a minute, when the plate being developed in the ordinary 
way three circular 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 desired, 
numerical values for the relative actinic powers of these sources of 
illumination under definite conditions; this I have, however, re- 
eretted the less, as the actual energy of the naked flames is not 
really the measure of their availability in Photo-micrography ; here 
the question of steadiness, involving, as it does, the possibility of 
great concentration, plays a most important part, and materially 
modifies the result. 
So far as I know, the Calcium light has never before been suc- 
cessfully employed as the source of illumination for making Photo- 
micrographs in this country. My friend Dr. R. L. Maddox, however, 
writes me that it has been experimented with in England by Drs. 
Abercrombie and Wilson. He thinks they used powers as high as 
an eighth with pleasing results. This mformation has directed 
my attention to the essay of Dr. Wilson in the ‘ Popular Science 
Review’ for 1867, volume vi., page 54, in which that gentleman 
gives in detail the process employed by himself and Dr. Abercrombie. 
He experimented with an oil-lamp and with the Oxy-calcitum and 
Magnesium lights. He says, page 59, of the Oxy-calcium light :— 
“T can scarcely think it would be used now that the more active 
light of Magnesium is within the reach of every one.” And of the 
Magnesium :—“ The light fails only in steadiness, and if some 
means could be devised for burning the metal uniformly and at a 
