PHOTOGRAPHY OF BACTERIA. 165 



gLiss replaced by a sensitive plate. To effect this change, the 

 operator retires to the dark-room, and opens a box of plates with 

 as little exposure to the red light as possible. Having removed 

 a plate, it is necessary to ascertain which is the sensitive side. 

 This may be done by momentarily exposing it to the red light, 

 and seeing which is the sensitive side by the dull appearance of 

 the film. A less satisfactory way is to moisten the tip of a 

 finger, and press it at one corner of the plate. The film side will 

 be recognised by imparting a sticky sensation. The film must be 

 dusted with a camel's-hair brush, as well as the dark-back, and 

 the plate is placed film-side downwards in the dark-back, which 

 is then securely closed. 



Care should be taken that the plates then remaining in the box 

 are packed away before light is admitted to the dark-room. 



Expos^ire of the Plate. — On returning to the apparatus, the 

 camera-shutter is closed. Then the dark-back is gently slid into 

 its place, and its slide -vvithdrawn. A few moments are allowed 

 to elapse, so that the least possible vibration, which might be 

 caused by inserting the dark-back, has had time to cease, and all 

 is ready for the exposure. 



In the case of the object we have selected, three seconds will 

 probably be the exposure required. This is done by opening and 

 closing the camera-shutter with one hand, while a watch can be 

 held in the other. The slide of the dark-back is then carefully 

 closed, and the plate is ready to be carried off to the developing 

 room. 



As the light will not be again required until the next exposure, 

 the oxygen must be turned off, while the coal-gas may be allowed 

 to play over the Hme. 



Development aiul Fixation of the Image. — It is well to be 

 systematic, and therefore, before the plate is taken out of the 

 dark-back, light is admitted to the dark-room, and everything 

 arranged so that the position of the trays and bottles may be 

 remembered in the dark. First, let the ruby lamp be Ut, place 

 two dishes or trays close by, and a row of four dishes within easy 

 reach. Pour out some fixing solution in the first porcelain dish, 

 alum in No. 2, and water in Nos. 3 and 4. Put the necessary 

 qu.antity of " pyro " solution into the glass measure, and place 

 it with the ammonia drop-bottle in front of the ruby Ught. 

 Then, when all light except that from the ruby lant-ern has been 

 excluded, everything is ready to commence the development of the 

 plate. 



