530- ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



flowing with iodide of potassium may be omitted, if the 

 plates are exposed for several hours to the sunlight. In 

 this condition they will keep indefinitely, and are rendered 

 sensitive by simply moistening them in the dark with dis- 

 tilled water, allowing a solution of tannin, of fifteen grains 

 to the ounce of water, to flow over them, and then drying 

 them as usual in the dark. They will retain their sensitive- 

 ness for months. The exposure required in the camera is 

 about the same as that for ordinary tannin plates, with the 

 pyrogallic-acid developer, and the negatives are said to be 

 cleaner. Instead of tannin, solution of nitrate of silver may 

 be employed, or simple "fuming" with ammonia-. Another 

 process of similar character has recently been suggested by 

 Krone, in which the action of light upon the sensitized plates 

 is prevented by taking advantage of the fact that iodide of 

 silver formed in the presence of excess of iodide of potash 

 is not sensitive to light, but can be rendered sensitive by 

 treating: it with nitrate of silver, etc. A collodion is there- 

 fore employed, containing nitrate of silver instead of an io- 

 dide, and the plate coated with it is dipped in a bath of io- 

 dide of potassium. It is then washed and treated with ni- 

 trate of silver in the dark, to render it sensitive, and dried. 

 The objection to this process is that it requires peculiar col- 

 lodion and baths, while the other requires only the ordinary 

 solutions. 4 D y July, 1874. 



OBJECT-GLASSES FOR PHOTOGRAPHY. 



In a paper on the transformation of the optical achroma- 

 tism of an object-glass into chemical achromatism, Cornu 

 finds that, given an achromatic telescope, the object-glass 

 of which is formed of a convergent lens of crown-glass and 

 a divergent lens of flint-glass, this object-glass may be form- 

 ed into one capable of giving satisfactorily distinct photo- 

 graphic images by separating the two lenses to an extent 

 dependent on the nature of the two glasses a method which, 

 by the by, was first introduced by Rutherford in 1850, but 

 subsequently abandoned by him for a better. With the 

 glasses used in optics a separation of 1^ per cent, of the focal 

 distance of the object-glass is sufficient, and the chemical 

 focus is then very near the optic focus. Using an excellent 

 telescope of about four feet focal length, and separating the 



