A Manual Of Photography

CHAPTER V. THERMOGRAPHY. The curious nature of the results obtained by heat radiations, associated as they are with the chemical action of the solar rays, induces me to introduce the subject in this treatise on Photo- graphy, merely reprinting my original communication on the subject, as the investigations have not been continued. The Journal of the Academy of Sciences of Paris, for the 18th of July, 1842, contained a communication made by M. Regnault from M. Moser, of Königsberg, “ Sur la Formation des Images Daguerriennes;" 1 in which he announced the fact, that " when two bodies are sufficiently near, they impress their images upon each other.” The Journal of the 29th of August contained a second communication from M. Moser, in which the results of his researches are summed up in twenty-six paragraphs. From these I select the following, which alone are to be considered on the present occasion :— “9. All bodies radiate light even in complete darkness. U10. This light does not appear to be allied to phospho- rescence, for there is no difference perceived whether the bodies have been long in the dark, or whether they have been just ex- posed to daylight, or even to direct solar light. "11. Two bodies constantly impress their images on each other, even in complete darkness. “14. However, for the image to be appreciable, it is neces- sary, because of the divergence of the rays, that the distance of the bodies should not be very considerable. "15. To render the image visible, the vapour of water, mer- cury, iodine, Ac., may be used. "17. There exists latent light as well as latent heat.” The announcement at a meeting of the British Association hf these discoveries, naturally excited a more than ordinary degree of interest. A discovery of this kind, changing, as it did, the features, not only of the theories of light adopted by philosophers, but also the commonly received opinions of man- 1 Comptes Rendus, tome xv., No. 3, folio 119.