NATURAL PHILOSOPHY. 127 



scientific interest. In order to a more full understanding of the subject, 

 we copy, from the London Art Journal, the memoir presented to the 

 French Academy, by M. Niepce, prefaced by some introductory remarks 

 by Robert Hunt', well known for his photographic researches. 



For the perfect understanding of the results obtained, it is necessary 

 that the chromatic conditions of a decomposed sunbeam should be 

 clearly appreciated and the relation of the colored image to the chemical 

 effects obtained distinctly understood. A pencil of light is passed 

 through a prism, and \ve obtain an elongated image consisting of a 

 beautifully colored set of bands. There are three primaries, red, yellow 

 and blue, which by intercombination give rise to other tints, so that 

 altogether we are acquainted with nine colored rays crimson, red, 

 orange, yellow, green, blue, indigo, violet and lavender. The colors 

 of natural objects are produced by the decomposition of the rays of 

 light, this be'ing effected by some peculiar surface action. Now, if we 

 expose a piece of photographic paper, or a daguerreotype plate, to the 

 action of this spectrum, or to the radiations from colored surfaces, we 

 shall find that the chemical effect has no relation to the intensity of 

 light belonging to the colored ray. For example, supposing the colored 

 image of the nine rays to fall upon a sensitive tablet, the result is of 

 the following curious character : One of the red rays protects the paper 

 from all change, the other usually makes a red impression ; the orange 

 and yellow rays have no chemical action, though these have the most 

 illuminating power. The chemical action commences in the green 

 ray, rapidly increases in energy in the blue, and exerts its maximum 

 power over the space covered by the indigo, violet, and lavender, still 

 continuing with much energy over a space beyond the lavender in 

 which no light can be detected. It was upon the consideration of these 

 peculiarities, clearly proving that ordinarily there was a remarkable 

 want of agreement "between the actinic power of the sunbeam and the 

 chromatic phenomena depending upon it, that M. Biot wrote the fol- 

 lowing passage : 



" Substances of the same tint may present, in the quantity or the 

 nature of the radiations which they reflect, as many diversities, or 

 diversities of the same order, as substances of a different tint ; inversely, 

 they may be similar in their property of reflecting chemical radiations 

 when they are dissimilar to the eye ; so that the difference of tint 

 which they present to the eye may entirely disappear in the chemical 

 picture. These are difficulties inherent in the formation of photo- 

 graphic pictures, and they show, I think, evidently, the illusion of the 

 experimenters who hope to reconcile, not only the intensity but the 

 tints of the chemical impressions, produced by radiation, with the 

 colors of the objects from which these rays emanate." 



These are the natural suggestions of tne mind when merely consid- 

 ering the ordinary phenomena of the chemical action of the solar 

 spectrum. But color is the result of a peculiar condition of the surface, 

 and if the different rays produce a dissimilar molecular or chemical 

 change, there is no reason why the result should not be the production 

 of chromatic impressions. The yellow rays produce a small amount 

 of chemical action, but that may result in such a molecular arrange- 



