PHOTOGRAPHING BACTERIA. 109 
to the technique of making photomicrographs, but append an account 
of a form of apparatus which we have used with great satisfaction : 
‘‘Photomicrography by Gaslight.—Those who have had much experience 
in making photomicrographs will agree with me that one of the most essen- 
tial elements of successis 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 are ready to. de- 
vote a day to making photomicrographs, the sun is obscured by clouds or 
the atmosphere is hazy. Indeed, in some latitudesand at certain seasons of 
the year a suitable day for the purpose is extremely rare. The use of sun- 
light 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 ex- 
tremely desirable. 
‘‘The oxyhydrogen lime light, the magnesium light, and the electric are 
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 
“lime light,’ and think it the best 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 power 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 photomicrographs of bacteria are 
made with an amplification of one thousand 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 colored screen must be interposed, 
which shuts off a large portion of, the actinic rays, on account of the staining 
agent usually employed in making my mounts. The most. satisfactory 
staining agents for the bacteria are an aqueous solution of fuchsin, or of 
methylene blue, or of gentian violet; and all of these colors are so nearly 
transparent for the actinic rays at the violet end of the spectrum thata 
satisfactory photographic contrast cannot be obtained unless we shut off 
these rays by a colored screen. 
‘‘T am in the habit of using a yellow screen for my preparations stained 
with fuchsin or methylene 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 tropeeolin dissolved in gelatin, 
‘‘But with such a screen, which 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 feeble- 
ness of the illumination. 
‘‘These considerations have led me to experiment with gaslight, and the 
simple form of apparatus which I am about to describe is the result of these 
experiments, I have now had the apparatus in use for several months, 
during which time I have made a large number of very satisfactory photo- 
riicrographs of bacteria from fuchsin-stained preparations with an amplifica- 
tion of one thousand diameters. My photographs have been made with the 
three-millimetre oil-immersion apochromatic objective of Zeiss and his pro- 
jection eyepiece No. 3. Iusea large Powell and Lealand stand, upon the 
substage of which I have fitted an Abbe condenser. The arrangement of 
the apparatus will be readily understood by reference to the accompanying 
figure. 
"A is the camera, which has a pyramidal bellows front supported by the 
