NATURAL PHILOSOPHY. 177 



ordinary circumstances, would disappear in a few minutes. Such is 

 the state of heliochromy to-day, and if the problem of fixation is not 

 yet solved, we may at least hope for a solution." M. Chevreul 

 remarks that the discovery of the dextrine and chloride of lead var- 

 nish is a great advance, and he compares the sensitive films of M. St. 

 Victor to the retina of the human eye. 



M. Chevreul, in commenting on these new results, called the atten- 

 tion of the Academy to the remarkable facts connected with them, 

 the first of which is, that the image produced by the sun is direct, 

 and not inverted, like those obtained by ordinary methods ; and the 

 second, that the light whitens the parts which it strikes, through a 

 special action of the dextrine and chloride of lead varnish, while 

 without this varnish it would impress a violet tint on the chloride of 

 silver of the daguerreotype plate, a remarkable result, since M. 

 Niepce has observed that the shadows of an engraving are repro- 

 duced in black on plates prepared with his varnish. The colors of 

 the image are not produced simultaneously ; for example, the yellow 

 appears before the green, and when this latter is manifested the yel- 

 low is weakened, if not effaced. Does it not follow from this that the 

 way to reproduce the colors with fidelity would be by the use of 

 screens, so arranged as to cover the parts where the colors are first 

 exhibited, so as to give more time to other colors which require it ? " 



THE PROSPECT OF OBTAINING PHOTOGRAPHS IN NATURAL COLORS. 



Sir David Brewster, in a recent publication, thus sketches the pro- 

 gress of heliochromy, and gives his views respecting the prospect of 

 the ultimate solution of the problem of fixing the colors of nature 

 upon a photographic picture. He says : 



More than one philosopher has expressed the opinion that the 

 finely-colored picture which appears with all the tints of nature on a 

 sheet of white paper placed in the camera can never be reproduced 

 and fixed either upon a paper or a metallic surface. This is the 

 principal discovery which science has in store for photography ; and 

 from the successful attempts which are making to reach it we are not 

 without hopes that it may yet be accomplished. 



In 1840 Sir J. Herschel obtained upon photographic paper a 

 colored image of the solar spectrum. Daguerre had previously 

 observed that a red house gave a reddish image on his iodized plate in 

 the camera; and Mr. Fox Talbot had observed that the red of a 

 colored print was red when transferred to paper washed with chloride 

 of silver. On paper washed with chloride of barium and nitrate of 

 silver, Mr. Hunt obtained red under a red glass, a dirty yellow under 

 a yellow glass, and a liylit olire under a blue glass. By preparing 

 metallic plates with chlorine, M. Becquerel obtained the spectrum in 

 colors, and also colored impressions of highly-colored maps; but, 

 though these colors were long durable in the dark, he never suc- 

 ceeded in fixing them. 



M. ISaepce St. Victor, setting out from the fine researches of M. 

 Becquerel, has been more successful by using the purest silver ; and 

 Mr. Hunt informs us that he has "examined pictures on metallic 

 plates, produced by Niepce, in which every color of the original was 



