CONCLUDING REMARKS.
Mist and haze are the great foes of successful telephotographic work and are not always easy to detect. An isochromatic screen to cut off only the ultra violet and violet-blue rays, with out reducing the brilliancy of the rest of the spectrum, will often enable a slight haze to be ignored. Vibration or shimmering of the atmosphere is another obstacle, difficult to overcome if present ; while the presence of wind is, of course, highly objectionable. If difficulty is found in focussing, a microscopic cover-glass may be attached with Canada balsam to the ground side of the screen. This will give an almost clear spot, on which a small portion of the image may be readily examined by the aid of a focussing magnifier. Unneces sary magnification should be avoided, a good rule being to start with a low power and grad ually raise this until the result is considered satisfactory.
powder used in this process a powdered vitrifiable colour is employed, the method immediately becomes applicable for cera mic purposes, since the picture will then stand firing without injury. This is, in
fact, the process chiefly in use. Another method, which is very similar, depends upon the fact that iron chloride, though not of itself adhesive, becomes so by ex posure to light. A third method is very like the carbon process, enamel colours being substituted for the ordinary pig ments, and a mixture of gum and honey for the gelatine. Fourthly, there is the substition process, in which a silver image obtained in the ordinary manner is converted into platinum or iridium by chemical substition, the latter metals being well adapted to stand extreme heat. In addition to these, there are one or two other methods of less importance, which will, however, be touched upon in due course,