NATURAL PHILOSOPHY. 145 



SO, the latter less visibly, — yet the bromide indicates a cnrrent of 

 even higher intensity than the former. The conclusion is, that a 

 precisely similar action takes place when the li<^ht acts on the 

 chloride and on tlie bromide of silver, n:i,mely, reduction to a sub- 

 chloride and subbromide respectively. On applying this curious 

 test to the iodide of silver, it was found that it likewise gave rise 

 to a current of positive electricity, under the influence of light, of 

 ijearly as high intensity as that afforded by the chloride. The 

 inference clearly is, that iodide of silver is reduced to a subiodide, 

 just as the chloride is to a subchloride, and the bromide to its 

 lower state of combination. 



Until recently all the evidence seemed to be tending to support 

 the purely mechanical theory of the formation of the latent image ; 

 latterly, the complexion of affairs has altered, and the evidence 

 tends in the direction of a chemical change as being the result of 

 the action of light, the experiments of M. Becquerel, referred 

 to above, forming a strong link in the chain. — British Journal of 

 Fhotograpliy . 



LUMINOUS PHOTOGRAPHS. 



One of the most curious inventions of the present day is the 

 new kind of photographs, made on a so-called phosphorescent 

 surface, of which absolutely nothing can be seen in the daylight, 

 but which are distinctly visible in the dark. Many years ago, 

 compounds were invented which had the property of shining in 

 the dark many hours, and even days or weeks, after an exposure 

 to sunlight for only a few seconds. These phosjohoric com- 

 pounds, called after their inventors, Canton's, Baldwin's, Bologn- 

 ian phosphorus, etc., were formerly of no use whatever, but it 

 was hoped that they might eventually reveal something concern- 

 ing the nature of light; and such has indeed been the case, as 

 the phenomena connected with these experiments are a strong 

 argument in favor of the undulatory theory, and the correlation 

 of forces. 



An English photographer lately conceived the idea of covering 

 a sheet of paper or glass with a layer of such a phosphorescent 

 substance, and then treating it in a similar manner to paper or 

 glass sensitized in the ordinary way for taking *a photograph. 

 Pictures taken in this way seem, by daylight, to have no existence, 

 but the jDlaces where the light has acted, become phosphorescent 

 or luminous in the dark, the shadows remaining invisible, the 

 semi-tints slightly luminous, and the result is such a change in the 

 surface that the picture is only perceptible in a dark room, by an 

 unearthly glow of a greenishl^ blue, red, or purplish tint, accord- 

 ing to the preparation used. 



It is very easy to make such pictures. A sheet of albumen 

 paper is moistened to make it sticky, and then equally covered 

 with a thin layer of the finely powdered phosphorescent substance, 

 or a pane of glass is covered with a thin coating of paraffine, to 

 which, also, when warmed, the powder will stick; then the pre- 



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