After this mixture has been carefully filtered through a paper filter, a few drops of ammonia are added. It will keep good for some time if well corked and preserved from exposure to the light. Even two months after being prepared I have found it to be still good; but too large a quantity should not be prepared at a time, as it does not improve with keeping. I find that the dry albumen of commerce will answer as well as the fresh. In that case I employ the following formula: Dry albumen from eggs.............. 15 to 20 parts Water.............................. 100 " Ammonia bichromate................. 2.50 " Always add some drops of ammonia, and keep this mixture in a well corked bottle and in a dark place. To coat the metal plate, place it on a turning table, to which it is made fast at the center by a pneumatic holder; to assure the perfect adhesion of this holder, it is as well to wet the circular elastic ring of the holder before applying it to the metallic surface. When this is done, the table may be made to rotate quickly without fear of detaching the plate by the rapidity of the movement. The plate is placed in a perfectly horizontal position, where no dust can settle on it; the mixture is then poured on it, and distributed by means of a triangular piece of soft paper, so as to cover equally all the parts of the plate. Care should be taken not to flow too much liquid over the plate, and when the latter is everywhere coated, the excess is poured off into a different vessel from that which contains the filtered mixture, or else into a filter resting on that vessel. The turning table should now be inverted so that the sensitive surface may be downwards, and it is made to rotate at first slowly, afterwards more rapidly, so as to make the film, which should be very thin, quite smooth and even. The whole operation should be carried out in a subdued light, as too strong a light would render insoluble the film of bichromated albumen. When the film is equalized the plate must be detached from the turning table and placed on a cast iron or tin plate heated to not more than 40° or 50° C. A gentle heat is quite sufficient to dry the albumen quickly; a greater heat would spoil it, as it would produce coagulation. So soon as the film is dry, which will be seen by the iridescent aspect it assumes, the plate is allowed to cool to the ordinary temperature, and is then at once exposed either beneath a positive, or beneath an original drawing the lines of which have been drawn in opaque ink, so as to completely prevent the luminous rays from passing through them; the light should only penetrate through the white or transparent ground of the drawing.