PHOTOGRAPHING BACTEPtlA. 113 



Plate Used. Orthochromatic plates only should be used. Of these I use 

 the Cramer rapid, isochromatic plate exclusively. With these when photo- 

 graphing bacteria and using an amplification of one thousand diameters the 

 exposure will vary from one and one-half to three minutes, two minutes 

 being about the average. 



It is with these plates that I have found a color screen unnecessary, and 

 since using them I have had 110 difficulty in photographing bacteria, for they 

 are particularly sensitive to the yellow-colored oil light. 



Possibly other makes of orthochromatic plates might be found to work 

 equally well, but the oil light works so very well with the Cramer isochro- 

 matic that I have had no desire to try others. 



Development. For development, I have obtained best results with for- 

 mulas in which hydrochinon either alone or with some other developing 

 agent is used. The following gives excellent results, and I prefer it to other 

 developers as it gives good clear negatives of sufficient contrast and 

 gradation : 



No. i. 



Water, ....... 10 ounces. 



Sodium sulphite, . ... 1 ounce. 



Potassium bromide, . . ... 10 grains. 



Hydrochinon, . . ... 30 grains. 



Metol, ... 40 grains. 



No. 2. 



Water, . ...... 10 ounces. 



Sodium carbonate, ... . 300 grains. 



Use equal parts of No. 1 and No. 2. 



Development should be continued until sufficient density is obtained. In- 

 tensification should be rarely required, for with proper exposure and develop- 

 ment a good negative can usually be obtained. If intensification is neces- 

 sary, after fixing and washing the plate, I prefer to use a saturated aqueous 

 solution of bichloride of mercury, followed by washing, the application of 

 dilute ammonia water, and a final washing. 



Students who desire to perfect themselves in the art of making 

 photomicrographs are advised to first make themselves familiar with 

 the technique of photography with a landscape or portrait camera, 

 and not to undertake the more difficult task of photographing bac- 

 teria until they know how to make a good negative and to judge 

 whether an exposure has been too long or too short, etc. 

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