The advance of photography : its history and modern applications (1911)

232 THE ADVANCE OF PHOTOGRAPHY He finds that the smaller the grain of the silver particle, the more minute is the variation in the stationary wave which can be recorded by the plate. It necessarily follows that the thicker the film, the greater will be the possible number of layers deposited ; and from this again results the greater purity of the reflected light. It is quite possible to obtain with the aid of the microscope photographs of the sections of the film, and so be able to count the number of laminae of deposited silver actually present. In some photographs of such films obtained by Neuhaus only seven or eight layers could be counted, but Ives has succeeded in obtaining photographs showing many more layers. There is a large amount of light lost at each lamina on account of the reflection and absorption which there takes place. As this is the case, the effect of each lamina depends very largely upon its distance from the surface, becoming rapidly less as this distance increases. This is what one would expect even if the layers were all equally well formed, but as a matter of fact they are not so, for the layers themselves also rapidly become less marked. This does not agree with the theory as worked out by Lehmann, for he found by calculation that the laminae should become more marked as their distances from the mirror are increased. He explained the want of agreement between theory and practice by assuming that the reflected light loses its powers of interference after a short distance. Ives has found that the purity of the reflected light is greatly increased when the amount of silver bromide used is diminished — in other words, as the grain becomes finer. He ascertained that this increase is most marked when quantities of silver nitrate between 0*18 gm. and 0*09 gm. are used for every gram of gelatine. The following is Ives' method of preparation : — A. Gelatine, 1 gram; water, 25 c.c.