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 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 are ready to de- 

 vote 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 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 oxyhydrogeii lime light, 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 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 tbat a 

 satisfactory photographic contrast cannot be obtained unless we shut off 

 these rays by a colored screen. 



" I am in the habit of using a yellow screen for rny 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 tropaeolin 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- 

 micrographs 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. I use a 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 



