with sensitometry and illumination, reflection and absorption, colorimetry, spectroscopy and geometrical optics. There will be a department of colloid chemistry, one of physical chemistry, one of organic chemistry, one of photo-chemistry to deal with the action of light upon a plate, and finally a number of photographic departments, dealing with photographic chemistry, with portraiture, color photography, photo-engraving, motion picture work and X-ray work, and the results obtained in all these departments will be applied first to the theory and then to the practise of photography.

In order to concentrate the different departments of the laboratory upon the photographic problems that arise and to ensure that on each problem the full knowledge and experience of the different specialists is made available, the main lines of work under investigation are discussed at a morning conference at the beginning of the day's work, one day of the week being assigned to each special subject, so that on Monday, for instance, those doing work in relation to one subject meet; on Tuesday the same men or other workers discuss a second aspect of the work of the laboratory, and so on. The laboratory organization, then, resolves itself into these several groups,

interlocked by their common members, who are dealing with a number of different lines of work.

The total work of the laboratory during the year may be represented by Fig. 3.

The departments of the laboratory are represented as circles on the outside of the chart, the main divisions in which problems group themselves being represented by the rectangles, subdivided in some instances, occupying the middle of the chart. Each of these rectangles corresponds to a morning conference; thus on Thursday mornings a conference is held on general photography, at which there are present members of the photographic department, the physics department, and the emulsion and coating or manufacturing departments. There is present at the conference, in fact, every scientific worker of the laboratory, whatever his rank, who is directly engaged on the subjects which are included under the head of general photography, and in some cases, or on special occasions, members of the staff of the company external to the laboratory are invited to these conferences, although it is not possible for many of them to be regularly present. All the main lines of investigation are laid down at these conferences and the progress from week to week carefully discussed. By the use of this system full cooperation and concentration of the different sections of the laboratory upon the problems to study which it has been founded is ensured.

Since the establishment of the laboratory, which was completed in 1913, a good deal of work has been finished and the foundations laid for much further research which can now be considered to be planned and arranged.

The work of the laboratory is published in the form of scientific papers, these being printed in the usual technical journals to which the special subject of the paper may be appropriate, and then at intervals, as sufficient papers accumulate, full abstracts of all the papers are collected and published in a volume under the title of "Abridged Scientific Publications." At the time of writing, October, 1917, about 65 papers have been completed.

The scientific work of the laboratory can be classified under the headings of the physics of photography, the chemistry of photography, the reproduction of tone values by photography, and work on special photographic processes, including those required for photography in natural colors. In addition to this a considerable amount of research has been done in pure chemistry and in the various branches of applied optics which are closely allied to photography.

THE PHYSICS OF PHOTOGRAPHY

Photographic sensitive surfaces do not consist of continuous coherent films of homogeneous material but have a definite granular structure, the sensitive material itself consisting of grains embedded in an insensitive matrix, so that in considering the properties of a sensitive photographic material we are considering really the properties of a collection of sensitive grains, which may differ considerably from each other in their individual properties. The properties of such a collection will be the statistical average of the individuals composing it and in order to understand the properties of a sensitive material we must therefore consider the properties of the individual grains and their relation to the aggregate material of which they are units.

The question at once arises: Do these grains consist of crystals of pure silver halide or of a gelatine silver complex? Microscopical study shows it to be probable that the grain is a pure silver halide crystal, for when these crystals are exposed to the action of water no swelling at all is observable even under the highest power of the microscope. The grains of silver bromide prove to be regular semi-transparent crystals belonging to the isometric system, occurring chiefly in triangular and hexagonal tablets and in needles of various thick

nesses, these needles being formed in the same way as the tablets (see Fig. 4). As they occur in a gelatine emulsion, these grains are doubly refracting, though this would not have been expected from their crystalline form (see Fig. 5).

Silver

bromide can be crystallized out from its solution in ammonia to show all the forms in which it occurs in emulsions, and the physical chemistry of the preparation of these crystalline grains is under investigation in the laboratory at the present time.

When the silver halide grains are developed, the crystalline form is lost, the silver being deposited in a sponge-form in sootlike particles, the form of the deposit being generally considerably distorted from the original shape of the silver bromide crystal grain, though in some cases the original shape is fairly well reproduced in the deposit of metallic silver.

In viewing a negative by transmitted light we can not, of course, see these isolated grains with the naked eye but we see a conglomeration caused by the penetration of light through the interstices between the grains distributed throughout the

emulsion layer, and thus we obtain regular large patches or chains of grain, the pattern and regularity depending upon the particular type of emulsion used. This granularity, the formation of which can be studied by the examination of a vertical section through the film, is what is meant by the "graininess" of photographic negatives in general and is the grain met with in enlarging, in projection, and in portraiture.

The granular structure of a photographic emulsion involves a limit to the resolving power of the emulsion; that is, it requires a certain finite distance between two points of light falling upon the film in order that they may record themselves as separate deposits of silver grains. The study of the resolving power of a photographic emulsion can be accomplished by the examination of the spread of the edge of an image. Suppose, for instance, that we lay upon a photographic film a knife edge and then illuminate this knife edge vertically from above; some of the light passing the knife edge will be scattered into the shadow by reflection from the grains of silver bromide and will produce developable grains within the shadow so that upon development we shall obtain a distinct extension of the image beyond the edge into the shadow. If we determine the relation between the number of grains rendered developable and the distance from the edge, we shall have a relation which will depend upon the scattering of the light by the silver bromide grains and upon the absorption of that light by the grains. These two factors we might term the "turbidity" and "opacity" of the emulsion.

An emulsion having high turbidity and low opacity will have a very low resolving power. On the other hand, even if the emulsion has high turbidity, if its opacity is also high, the resolving power may be good. A typical example of this is the wet collodion plate, in which the turbidity is considerable but the opacity of the silver iodide for blue-violet light is so great that the resolving power is high. In the grainless Lippmann emulsion the resolving power is high if the emulsion is very clear, because the turbidity is very small, but the opacity is also small so that the slightest increase in turbidity may make the resolving power very low.

A convenient way of measuring resolving power is to photograph a converging grating, observing the point in the photograph at which resolution first occurs, from which a numerical measurement of the resolving power can be obtained.

The importance of photographic resolving power in relation to many branches of scientific work and especially spectro