ii4 PHOTOGRAPHY FOR NATURALISTS. 



half-an-hour, or to diffused light for several hours. The 

 plates keep indefinitely ; indeed improve by keeping ; 

 and their coated sides can easily be distinguished by 

 touch. An enormous number can be done in half-an- 

 hour, if they are cleaned and ready beforehand. 



For the best possible results polished plate should 

 be used, but old negative glass will answer all ordinary 

 purposes. The great advantage in the use of polished 

 plate, apart from its translucency, arises from its equality 

 of surface, and consequent close contact with the tissue. 

 Ordinary, as opposed to plate-glass, is always slightly 

 curved. The best method of cleaning negative glass, 

 after getting rid of the emulsion, is to soak it in a very 

 weak solution of ammonia. Plates must be selected 

 which are free from serious defects in the way of bubbles 

 or scratches. 



The glasses being coated, and a supply of sensitive 

 tissue at hand, printing proceeds by the aid of an 

 actinometer. There is not much to choose between 

 the various actinometers on the market. It must be 

 borne in mind, however, that the term " standard tint " 

 is misleading, owing to the fact the colour acquired by 

 P.O. P. on exposure to light varies, not only with the 

 brand employed, but also with the state of the 

 atmosphere. Consequently, a tint which will match a 

 certain stage of exposed P.O. P. on one day will not 

 necessarily do so on a future occasion. In practice 

 this makes very little difference, as one gets to judge by 

 the P.O. P. itself without troubling about the standard 

 tint. 



