International Review of Educational Cinematography (Jul-Dec 1929)

series of images intended to supply the monochrome red and on the other the monochrome green series. This does not involve any very arduous mechanical problem, but it is obviously necessary to have a printing machine which allows the negative films to be moved at just double the speed of the rough films. The mechanical exigencies of positive printing may on the other hand differ according to the method used in selection, while some systems, such as the Audiberc system described by Prof. Seyewetz in the 4th number of the Review, may entail the necessity of enlarged prints. The other difficulty, which is of a physical kind, derives from the necessity of preventing the light, while a series of images is being impressed on one stratum, from traversing the celluloid ribbon, and producing an impression also on the stratum on the opposite side. This latter difficulty is completely eliminated by dyeing the emulsion yellow; thus for instance a slight degree of colouring with tartrazine yellow removes the difficulty without affecting the print, while the yellow colouring disappears entirely in the course of developing, fixing and washing the film. Or again a stratum of hydrate of bi-oxide of brown manganese, precipitated in the emulsion, renders the stratum quite opaque, while the manganese composition can very easily be removed by using a solution of hyposulphite containing a sufficient quantity of bi-suiphite of sodium for fixing. The development of double-faced films calls also for some special arrangements which are quite easily made. By recourse to the methods above indicated, we obtain a positive film bearing a double series of black images, one on either surface; each of these series of images must afterwards be converted into the corresponding monochromes, How the Images are converted into Monochromes. It is not uncommon nowadays to speak of the transformation of a black image (formed as is known by reduced silver) into an 684