82 MICRO-ORGANISMS IN WATER 



The above manipulations are carried out in the dark 

 room, and when all is ready the apparatus is placed in 

 the light, the length of exposure being varied according 

 as a dark or light toned photograph is desired. In 

 strong sunshine usually half a minute's exposure was 

 found to yield the best results. The print is further 

 treated in the usual manner. The paper is removed to 

 a dark room, and washed repeatedly to get rid of the 

 excess of silver ; it is then placed in a chloride of gold 

 bath, and then in one of sodium thiosulphate, in which 

 it is left until it has become well fixed, when after wash- 

 ing again it is finally dried. 



Giaxa states that in this manner he has been able 

 to reproduce with the greatest ease the characteristic 

 appearances of various plate-cultures. 



In some cases the possibility of permanently record- 

 ing such appearances may be of much value and interest ; 

 whilst a collection of such photographic prints of plates 

 might often afibrd a most useful record of the broader 

 and more striking bacterial differences between various 

 waters, and thus become of great service for purposes 

 of reference. 



