The sensitising solution is made up of gelatine and bichromate of potash dissolved in water; before use it is filtered, freed from air bubbles and heated to not more than 120 deg. F. The plate is now placed on a stand, which is provided with levelling screws, in the oven, and, when the temperature has reached 120 deg. F., an amount of the bichromate gelatine solution sufficient to make a thickness of film proper for the mode of printing to be employed is poured upon the plate. The oven is kept at a constant temperature, 120 deg. F., until the gelatine is dry, when it is allowed to cool gradually. Whilst the gelatine is setting, precautions against vibration must be taken else the plate will be spoilt. When dry, the collotype plate is sensitive to light and moisture; its surface shows a more or less regular series of convolutions which resemble those of the outer surface of the human brain, although, of course, very much smaller. The character of the grain is very important, for if it be too fine it will not take up a sufficiency of ink, and, on the other hand, if too coarse it will yield coarse impressions. A reversed negative, of a quality beyond reproach, must be made of the original; if the subject is dark or has heavy shadows the negative is frequently slightly over exposed so as to soften them. The collotype plate is then exposed under the negative and washed in cold water until the yellow bichromate no longer comes away. It is then dried. In printing, the plate is damped and rolled up with ink as in lithographic printing; the amount of ink adhering to the film depends on the extent to which the different parts have been acted on by the light, as has already been mentioned. The moistening of the plate--mis-termed etching--is best done with dilute glycerine containing 75 per cent. of water, which when first applied should be allowed to remain on for about half-an-hour. The excess of moisture is taken up with a sponge or a ball of rag, and then the plate is inked and printed in a lithographic or a collotype press. The picture is usually masked with tin foil in order that its edges may be quite clean. Of the faults which may occur, the following may be alluded to. A mottled appearance may obtain in the high lights; this is due to the coating of gelatine being too thick. More commonly, the reproductions may appear flat owing to the degradation of the high lights; this is a sign that the sensitive film has been acted upon by moisture during its critical existence between the drying and the washing out of the potassium bichromate, or that the temperature has been too low. The following contain good examples of collotype. Karsten and Schenck: _Vegetationsbilder_, Jena. Oliver: _Notes on Trigonocarpus and Polylophospermum_. New Phytologist, Vol. 3, 1904. Semon: _Zoologische Forschungsreisen in Australien_. Jena. 1912.