A Manual Of Photography

144 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. placed near the window of a well-lighted room, and so arranged that, during the whole time of the preparation of the plate, by exposure first to iodine and afterwards to bromine, it was ex- posed to full daylight, and by a mechanical arrangement, of too obvious a nature to render description necessary, the plate was withdrawn from the bromine vessel into a dark box ; that is to say, it was withdrawn at the same moment from the influence ot both light and bromine : on being placed in the camera obscura, plates so prepared received impressions which by mercurialisation produced excellent pictures, and there was no trace ot the action of any light save that of the camera obscura. It follows, then, that light is incapable of exerting any appreciable influence on daguerreotype plates during the time they are receiving then- coatings of iodine and bromine., “ Although these experiments afford no information on the subiect in reference to which they were originally undertaken, they are yet not without interest, both in their theoretical bearing and in their practical application. They demonstrate not only that the change (whatever it may be) effected by light on silver plates prepared by Daguerre’s process is completely suspended in the presence ot the vapour ot either iodine or bro- mine, but that after that change has been produced the impres- sion may be destroyed, and the plate restored to its original condition, by a momentary exposure to either of these vapours. In their practical application, these experiments show that all the care which has been taken to exclude light from daguerreo- type plates during their preparation is unnecessary ; that so far from a dark room being essential to the operations of the da- guerreotype artist, the light of day may be allowed to fall on the plate during the whole time of its preparation; and that it is onlv necessary to withdraw it at the same moment from the action of bromine and light by sliding it from the bromine vessel into the dark box in which it is carried to the camera obscura; and where, from the situation or otherwise, there is a difficulty in observing the colour of the plate during the process of iodizing, it may be removed from the iodine vessel, and its colour examined by the direct light of the sun, without risk or injury: for when returned to the iodine or bromine vessel for a moment, the effect of light is wholly destroyed. “Perhaps the most valuable practical application ot these tacts is in the use of the same plate for receiving several impressions. When, on taking the portrait or picture ot any object liable to move, there is reason to suppose that the motion ot the person or object has rendered the operation useless, it is not necessary to throw aside the plate on whieli the imperfect impression lias