those derived from the " cubic " system. The most general shapes are triangular or hexagonal tablets of thicknesses between and i/r5th of their diameter. The large flat faces arrange themselves, during drying, parallel to the free surface of the gelatine. Microscopic examina tion in polarized light shows double refraction, indicating the existence of internal strains which extend to the gelatine surrounding the grains.
The mean and maximum dimensions of the grains vary considerably with the emulsion. In the so-called " grainless " emulsions (very slow emulsions, not made commercially but used in certain cases where extreme fineness of the image is necessary) the mean diameter of the grains is about o-0002 mm. (two ten-thousandths of a millimetre), whilst in ultra-rapid emulsions (about r5o,000 times more rapid than grainless emulsions) the mean diameter of the grains is from 0-003 to 0-004 In a single emulsion, the grains have very different sizes, even in cases where the emulsion is not made by mixing two emulsions prepared separately. As a general rule the differences in dimensions between the extreme grain sizes becomes greater as the emulsion becomes more sensitive. In a single emulsion the large grains are always more sensitive than the small These grains are not usually of the pronounced forms and sharp edges of crystals formed freely in the absence of gelatine ; in fact, they are not made up simply by the silver halides, but contain also a certain quantity of gelatine and water.
The thickness of a layer of emulsion on negative plates and films in common use is generally from 0-o3 to o-o4 millimetre. The grains form several superposed layers, and it is impossible to see the grains individually in the microscope, even under strong magnification. What is actually observed is due to the fact that the emulsion sometimes contains agglomer ations of grains almost in contact, and that grains situated at different depths in the layer overlap one another. It is this fact which gives rise to the lack of homogeneity of photographic images (Fig. 137) seen under the largest magni fications used in practice (such as enlargements and projections). These magnifications are, however, very much less than those necessary to show the individual grains. This lack of homogeneity is called the graininess of the image, and generally becomes more apparent for the more sensitive emulsions having bigger grains. In much of what follows grain and
graininess must not be confused by the reader.
The free surface of the dried image, seen by reflection, appears the brighter as the grains of silver there arc finer and rarer. The whites of a slow emulsion, clearer than those of an image with a rapid emulsion, are always brighter. Superficial reduction, dissolving the grains nearest the free surface, leaves a uniformly bright surface.
197. The Latent Image. A very sensitive emulsion, fully ripened, drops enormously in sensitivity, and may even revert to the sensi tivity it had before ripening, when it is subjected to the action of oxidizing agents, which, on the contrary, have no action on non-ripened or only slightly-ripened emulsions. Thus it would seem that the great sensitivity of ripened emulsions is due to some other substance than silver bromide. This substance, in the form of nuclei distributed amongst the grains according to the laws of chance, is not sensitive itself but accelerates the formation of the image.' On exposure to light these nuclei take part in some change which occurs, giving rise to latent image centres, the points from which start the reduction of the silver bromide on development. These points have been shown experimentally by T. Svedberg (1922). The photo-micrograph in Fig. 138 shows in the first picture the grains in a diluted emulsion after exposure before development, in the second picture the centres after very short development followed by fixation, and in the third picture (by super position of the first two), the position of the centres in the grains.
It has been directly shown that if silver bromide is exposed long enough to an intense light it is partially decomposed into metallic silver and free bromine, but under the conditions normally prevailing in the formation of the photographic image it is impossible to prove directly that this takes place. This is because the total mass of substance decomposed is considerably less than the least which can be detected either by the most delicate methods of chemical analysis, or by microscopic examina tion under the highest practicable magnification. Thus we can only make assumptions as to the nature of the latent image.