PHOTOGRAPHY OF BACTERIA. 167 



and then placed in dish No. 2. When another plate is ready to 

 take its place, transfer it to dish No. 3, and then to No. 4, and, 

 after a good final washing under the tap, place it upon a rack 

 to dry. If there is any tendency for the film to detach itself from 

 the plate, " to frill," the alum bath must be used before fixing, as 

 well as after. 



Frilling or blistering may be due to an error in manufacture, 

 and. is liable to occur in hot weather, or when using a developer too 

 strong in ammonia. If it occurs during washing or fixing, the alum 

 bath must be employed before the hypo. Fogging, or the appear- 

 ance of a veil over the plate, may arise from error in the manu- 

 facture, from admission of extraneous light, from over-exposure, 

 or from prolonged exposure to the ruby light during development. 

 Care must be taken that the camera and dark-room are light-tight. 

 Crystallisation, or powdery deposit, upon the negative when dry, is 

 due to insufficient washing out of the hyposulphite of soda. Thin- 

 ness of the image, or want of density, may be due to insufficient 

 development, too weak a developer, or too short or too long an 

 exposure. Too great density results from too long immersion in the 

 developer. 



Spots may sometimes occur upon the negatives. They may be 

 caused by dust upon the plate or by air bubbles in the developer. 



In the text-books of photography full accounts of failures will be 

 found, their causes and prevention ; but it will be advantageous 

 when these difficulties are encountered to take the negatives to a 

 skilled photographer and get advice upon them. It is necessary 

 to persevere, and not be disheartened if several negatives have to 

 be made of a preparation before a successful result is obtained. 



It may here be remarked that the beginner will far more 

 rapidly learn the technique if he avail himself of a practical demon- 

 stration from a photographer. When he has learnt to obtain suc- 

 cessful negatives, if he prefer silver prints, and time is an object, it 

 will be found to be true economy to get the printing and mounting 

 done by a professional photographer. The credit of a successful 

 photograph of bacteria is due to the bacteriologist who prepares 

 the microscopical specimen and obtains the negative. 



Determination of the Amplification. The amplification varies 

 not only with the objective employed, but with the distance of 

 the focussing screen from the object. In order to ascertain the 

 amplification afforded by a certain objective at a certain distance, a 

 photograph should be taken, under the same conditions, of the lines of 

 a micrometer slide. It is easy then to calculate the amplification 



