reception of the tympan; =M=, large clamp for stretching the elastic sheet =O=; =N=, zinc tympan, greased to facilitate its movement under the scraper, it also prevents grease penetrating the sheet =O=; =P=, a bracket on bed =A=, against which the tympan frame rests when raised and thrown back. The arrangement is used as follows: When the plate has been satisfactorily inked or rolled in, the slide or bridge =B= is adjusted over the plate, leaving a strip of about three _mm_. uncovered to allow of adjustment of the scraper. Lower the frame =D= in which the zinc strips have been adjusted to the size of picture; the printing paper is placed over this slide, and covered with a few sheets of paper backing. The tympan frame =L=, together with the sheeting and greased zinc plate, are lowered upon the work, the scraper adjusted, and an impression taken. [Illustration: _Fig. 7._] CHAPTER V. CHEMICALS AND MATERIALS FOR COLLOTYPE. =Gelatine.=—The basis of the Collotype process is gelatine—the finest purified animal glue—and the important part it plays in the process demands an acquaintance with its properties, which are found to differ widely, according to its origin and method of preparation. Gelatine suitable for Collotype should be free from all impurities—particularly alum, which is not infrequently added in small quantities during manufacture. Dr. Eder, in particular, has made most minute examinations of various samples, more especially with regard to their suitability for Collotype, Carbon, and the Galvanographic processes, and his opinion, agreeing with that of other experimentalists, is here quoted:—