A Manual Of Photography

264 PRACTICE OF PHOTOGRAPHY. which also assists in getting rid of the ether from the surface, for without this is thoroughly done a uniform coating cannot he obtained; but on no account should it be removed until the plate has been immersed about half a minute, as marks are apt to be produced if removed sooner. Having obtained the desired coating, the plate is then extremely sensitive, and, therefore, we presume the operator has taken every precaution to exclude ordinary day-light. The room must be closed against any portion of day-light, and candle alone employed, placed at a distance from the operator to give the requisite light. The plate thus rendered sensitive must then be lifted from the solution and held over the trough, that as mucli liquid as possible may drain off previous to being placed in the frame of the camera, and the more effectually this is done the better, or the action in camera will not be equal over the whole surface ; at the same time it must not be allowed to dry, but, in short, to obtain its ful1 maximum of sensibility, it should be damp without superfluous moisture. The question, says Mr. Horne, is often asked, how soon after coating the plate with collodio-iodide should it be immersed in the nitrate batli ? Now, this is a difficult question to answer. We have said the time of immersion is dependent upon the temperature and quality of the collodion ; so likewise must we be governed as to time before immersion. To make collodio-iodide or xylo-iodide, for, chemically speaking, there is no difference in the two, it is necessary that the ether should contain a certain quantity of alcohol, or the different articles are not soluble: therefore, if we take a fresh bottle, and coat the plate from this, it contains its full dose of ether, and with the thermometer ranging between 60° and 70° the evaporation of this article will be very rapid, and consequently a tough film soon formed. If on the other hand we are using a solution which has been in use some time, and many plates, perhaps, coated, the proportion of alcohol is much greater, and not being of so volatile a nature, it will necessarily take a longer time to acquire the requisite firm- ness for immersion. If, for instance, after coating a plate, we find on immersion it does not colour freely, we have then reason to suppose the plate has not been immersed sufficiently quick, but if on the other hand we find the film very tender, and upon drying it cracks, then we have reason to know that plates prepared from that bottle must not be immersed quite so soon. The larger the proportion of alcohol the more sensitive will be the plates, and the (quicker and more even will be the action of bath. but a longer period must be allowed for the sensitive film to harden before immersion.