A Manual Of Photography

168 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. glass were well made out, and a remarkably strong impression where the crown glass rested on the tracing paper, hut the mica had not made any impression. 10. The last experiment repeated. After the exposure to mercurial vapour, heat was again applied to dissipate it: the impression still remained. 11. The experiment repeated, hut the vapour of iodine used instead of that of mercury. The impressions of the glasses ap- peared in the same order as before, but also a very beautiful image of the mica was developed, and the paper well marked out, showing some relation to exist between the substances used and the vapours applied. 12. Placed the glasses used above, with apiece of well-smoked glass, for half an hour, one twelfth of an inch below a polished plate of copper. The vapour of mercury brought out the image of smoked glass only. 13. All these glasses were placed on the copper, and slightly- warmed : red and smoked glasses gave, after vaporisation, equally distinct images, the orange the next, the others left but faint marks of their forms ; polishing with Tripoli and putty powder would not remove the images of the smoked and red glasses. 14. An etching, made upon a smoked etching ground on glass, the copper and glass being placed in contact. The image of the glass only could be brought out. 15. A design cut out in paper was pressed close to a copper plate by a piece of glass, and then exposed to a gentle beat ; the impression was brought out by the vapour of mercury in beauti- ful distinctness. On endeavouring to rub off the vapour, it was found that all those parts which the paper covered amalgamated with mercury, which was rubbed from the rest of the plates: hence there resulted a perfectly white picture on a polished copper plate. 16. The coloured glasses before named were placed on a plate of copper, with a thick piece of charcoal, a copper coin, the mica, and the paper, and exposed to fervent sunshine. Mercurial vapour brought up the images in the following orders : smoked glass, crown glass, red glass, mica beatifully delineated, orange glass, paper, charcoal, the coin, blue glass : thus distinctly proving that the only rays which had any influence on the metal were the calorific rays. This experiment was repeated on different metals, and with various materials, the plate being ex- posed to steam, mercury, and iodine: I invariably found that those bodies which absorbed or permitted the permeation of the most heat gave the best images. The blue and violet rays