again by candlelight with gallo-nitrate of silver, and warm them : this causes all the shades of the picture to darken greatly, while the white parts remain unaffected. The shaded parts of the paper thus acquire an opacity which gives a renewed spirit and life to the copies, of which a second series may now be taken, extending often to a very considerable number. In reviving the picture it some- times happens that various details make their appearance which had not before been seen, having been latent all the time, yet ne- vertheless not destroyed by their long exposure to sunshine.
The author terminates these observations by stating a few expe- riments calculated to render the mode of action of the sensitive paper more familiar.
1. Wash a piece of the iodized pa'per with the gallo-nitrate ; ex- pose it to daylight for a second or two, and then withdraw it. The paper will soon begin to darken spontaneously, and will grow quite black.
2. The same as before, but let the paper be warmed. The blackening will be more rapid in consequence of the warmth.
3. Put a large drop of the gallo-nitrate on one part of the paper and moisten another part of it more sparingly, then leave it |ex- posed to a very faint daylight; it will be found that the lesser quantity produces the greater effect in darkening the paper ; and in general, it will be seen that the most rapid darkening takes place at the moment when the paper becomes nearly dry ; also, if only a portion of the paper is moistened, it will be observed that the edges or boundaries of the moistened part are more acted on by light than any other part of the surface.
4. If the paper, after being moistened with the gallo-nitrate, is washed with water and dried, a slight exposure to daylight no longer suffices to produce so much discoloration ; indeed it often produces none at all. But by subsequently washing it again with the gallo-nitrate and warming it, the same degree of discoloration is developed as in the other case (experiments 1 and 2). The dry paper appears, therefore, to be equal, or superior in sensitiveness to the moist ; only with this difference, that it receives a virtual instead of an actual impression from the light, which it requires a subsequent process to develope.
5. " New mode of preparation of the Daguerreotype plates, by which portraits can be taken in the short space of time of from five to fifteen seconds, according to the power of light, discovered by A. Claudet in the beginning of May 1841." Communicated by the Marquis of Northampton, Pres. R.S.
" My improvement," says the author, " consists in using for the preparation of the plates, a combination of chlorine with iodine, in the state of chloride of iodine. I follow the preparation recommended by Daguerre. After having put the plate in the iodine box for a short time, and before it has acquired any appearance of yellow co- lour, I take it out, and pass it for about two seconds over the open- ing of a bottle containing chloride of iodine ; and immediately I put it again in the iodine box, where it acquires very soon the yellow
