ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
667 
Photomicrography by Gaslight.* — Major Geo. M. Sternberg 
observes : — Those who have had much experience in making photo- 
micrographs will agree with me that one of the most essential elements 
of success is the use of a suitable source of illumination. 
Without question the direct light of the sun reflected in a right line 
by the mirror of a heliostat is the most economical and in some respects 
the most satisfactory light that can be used. But we cannot command 
this light at all times and places, and it often happens that when we aro 
ready to devote a day to making photomicrographs the sun is obscured 
by clouds, or the atmosphere is hazy. Indeed, in some latitudes and at 
certain seasons of the year a suitable day for the purpose is extremely 
rare. The use of sunlight also requires a room having a southern 
exposure and elevated above all surrounding buildings or other objects 
by which the direct rays of the sun would be intercepted. For these 
reasons a satisfactory artificial light is extremely desirable. 
The oxy-hydrogen limelight, the magnesium light and the electric 
arc light have all been employed as a substitute for the light of the sun, 
and all give satisfactory results. I have myself made rather extensive 
use of the “ limelight,” and think it the b st substitute for solar light 
with which I am familiar. But to use it continuously, day after day, is 
attended with considerable expense, and the frequent renewal of the 
supply of gas which it calls for is an inconvenience which one would 
gladly dispense with. 
These considerations have led some microscopists to use an oil lamp 
as the source of illumination, and very satisfactory photomicrographs 
with comparatively high powers have been made .with this cheap and 
convenient light. But in my experience the best illumination which I 
have been able to secure with an oil lamp has called for very long 
exposures when working with high powers, and as most of my photo- 
micrographs of bacteria are made with an amplification of 1000 diameters, 
I require a more powerful illumination than I have been able to secure 
in this way. And especially so because of the fact that a coloured screen 
must be interposed, which shuts off a large portion of the actinic rays, 
on account of the staining agents usually employed in making my mounts. 
The most satisfactory staining agents for the bacteria are an aqueous 
solution of fuchsin, or of methyline-blue, or of gentian-violet, and 
all of these colours are so nearly transparent for the actinic rays at the 
violet end of the spectrum that a satisfactory photographic contrast 
cannot be obtained unless we shut off these rays by a colour screen. 
I am in the habit of using a yellow screen for my preparations stained 
with fuchsin or methylen-blue, and have obtained very satisfactory 
results with the orthochromatic plates manufactured by Carbutt of 
Philadelphia, and a glass screen coated with a solution of tropoline 
dissolved in gelatin. 
But with such a screen, w r hick shuts off a large portion of the actinic 
light and increases the time of exposure three or fourfold, the use of an 
oil lamp becomes impracticable, with high powers, on account of the 
feebleness of the illumination. 
These considerations have led me to experiment with gaslight, and 
* John Hopkins University Circulars, ix. ( 1890 ) p. 72 . 
