EDWARD ELS WORTH. 175 



mercurial mirror behind the plate has caused the red 

 light to deposit the silver in layers corresponding to the 

 length of a red wave, etc. These layers coinciding ex- 

 actly Avith the length of a certain wave of light, can only 

 let pass the same light which originated them. 



It is the principle of the phonograph applied to light 

 waves instead of sound waves. 



So far as any practical use that can be made of Lipp- 

 man's process is concerned, it may be said that it is 

 merely suggestive. As a scientific experiment it is in- 

 teresting and some future explorer may, through its 

 channels, develop) a process of color photography. 



Up to this date it has been used only to record the 

 colors of the spectrum, as represented by rays of light 

 passing through stained glasses. 



It might produce a negative plate for lantern pro- 

 jection, but at best the process is inconvenient for prac- 

 tical use. 



To my mind one great objection to the process is the 

 time of exposure necessary ; the period required for the 

 impression of the red waves must certainly obliterate 

 the blue, resulting in contrasts which must yield a bad 

 positive. 



In short the achievement of Lippman, which so ex- 

 cited our Parisian and English photographers last year, 

 is little more than was accomplished by Ritter in 1801, 

 by Seebeck in 1810, and by Herschel in 1839, even be- 

 fore the present art of photography was known. 



Speaking of these experiments the late Mr. D. Wis- 

 tanley, in an article published in a recent number of 

 ''Photography," an English magazine, said " I think 

 that all these experiments with the solar spectrum are 

 experiments in the wrong direction, for in making 

 photographs of such objects as we see around us, we 

 very seldom indeed have pure spectrum colors with 

 which to deal. The thing which looks red only does so 



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