498 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



ures will be found impressed upon the collodion when the 

 'paper is removed, the design remaining perfectly visible aft- 

 er the complete drying of the film. It is suggested that this 

 may be the germ of some important applications in the way 

 of fhe reproduction of printed matter without injury to the 

 original. 6 A. June 3, 686. 



GLASS FOR PHOTOGRAPHING. 



Photographers have long been aware that common glass is 

 better adapted, as far as clearness is concerned, for receiving 

 several successive negatives than mirror or plate glass, not- 

 withstanding: the difference in the evenness of the surface. 

 This is said to be due to the fact that the speedy cooling of 

 the surface of glass develops a very hard external skin or 

 layer, the pores of which are extremely compact, this coat- 

 ing being removed in the process of grinding plate-glass. If 

 a negative be made upon a plate of ground glass, and after- 

 ward apparently entirely removed, it will often happen that, 

 in attempting to print from a second negative, the figures of 

 the first one will be likewise reproduced, sometimes with re- 

 markable clearness, although not the slightest trace may "be 

 visible to the eye. This accounts, in some, if not all cases, 

 for the so-called " spirit photographs" which have occasion- 

 ally perplexed, and even terrified operators. Common glass, 

 on the contrary, by the compactness of its pores, resists the 

 absorption of the silver, and permits the original picture to 

 be entirely removed. It is found, too, that in many instances 

 thin colored liquids will be absorbed in the^surface of ground 

 glass so that they can never be removed, a condition which 

 does not occur with the common kind. 



A glass is now prepared in Liverpool, according to the 

 photographic journals, which is free from the defects in ques- 

 tion. Although it is blown, yet this is done with very great 

 care, and the surface is afterward very carefully and thor- 

 oughly polished by appropriate machinery without removing 

 the external skin referred to. 6 C,June 1, 215. 



TAPIOCA PAPER IN PHOTOGRAPHY. 



A substance called tapioca paper recommended as very 

 useful in taking photographs by artificial light is prepared 

 by soaking 300 grains of tapioca for 2 days in an equal weight 



