Scientific American 20, 12.5.1860
It is certainly wonderful that two of the finest colors known - namely, the purple of the ancients and the celebrated Chinese green, or lokao - i. e., the finest color furnished by the animal kingdom, and the finest color furnished by the vegetable kingdom - are produced by the direct agency of light. At the last meeting of the Academy of Sciences, M. Lacaze-Duthiers read a paper upon the production of the Tyrian purple, and has again called attention to the wonderful part light plays in the formation of this color - a fact that has long been well known. The only thing really novel in this long dissertation is the description by its author of the organs which, in certain marine molluscs, secrete the colorless liquid that finally turns to purple when exposed to the air and to daylight. This organ is nothing more than a small cluster of cells, situated at the surface of the animal's body, and quite distinct from the corpus bojani, or kidney of gasteropodous mollusca. The product secreted by this organ in the genera, purpura and murex, is a colorless, whitish, or slightly yellow liquid, which is extremely photogenic. "The action of light upon this liquid," says our author, "has for effect the development of the three simple colors, yellow, blue and red; between which are observed, as effects of mixture, green and violet. When the experiment is made in diffused daylight - that is to say, slowly - the order in which the colors appear is observed in a very perfect manner. But whilst the yellow disappears as the action of the light continues, the blue remains constantly in a certain quantity, so that the red is never to be obtained alone, and the purple produced by these natural means is always more or less violet." M. Lacaze-Duthiers, has, moreover, experimented photographically with this Tyrian purple; he has obtained proofs upon silk batiste, &c., which, although they do not offer the perfections of ordinary photographs, present, nevertheless, in the numerous details, a great strenght of tone. In an image thus obtained, we again meet with the colors above named: a greenish yellow corresponds to the white parts, and a more or less dark violet to the dark portions, or ordinary photographic proofs.
- London Photographic News.