On looking at the exposure table on p. 91 we find that the normal exposure given for the particular conditions of light is, say, sec. Withdrawing the slide-shutter, we expose for half that time, or sec. ; then, replacing the shutter of the slide over one quarter of the plate, we expose again for another ; pushing it in again to halfway, a third exposure is given, and then a final, with the result that we have secured on one plate a record of four exposures for A-, 11o, and about sec. respectively. On developing these for a fixed time we shall be able to decide from which of these exposures we can obtain the relative factor for this brand of plates.
Quality of Light.—The chemical intensity of the light is, unlike the other factors, a variable one, and quite inde pendent of the will of the operator. Conditions of atmo sphere, geographical latitude, and the season of the year have to be considered, as well as the altitude of the sun above the horizon. The most intense light is present on a sunny, clear day for about an hour before noon ; after wards the evaporation caused by the heat of the sun inter cepts a portion of the light. The lower the sun is in the heavens, the thicker and denser the layers of atmosphere through which his rays have to pass. Clouds and mist seriously decrease the amount of available light ; dull, heavy clouds reduce it sometimes to about one-eighth that of a clear day at the same time of year.
Actinometer.—A simple actinometer for measuring the chemical intensity of light at a particular time may be devised as follows : Soak a piece of bromide paper of any good make in a io per cent. solution of potassium nitrate. This operation and the subsequent drying must, of course, take place in yellow light. Take a manilla thick envelope and cut a small hole about inch square in one face of it. Expose a slip of this specially prepared paper to reflected light for a period sufficient to give it a definite blue tint, then with water-colour or pencil surround the square aperture with a border exactly matching this tint. A constant fraction of the time which the sensitive paper takes in reaching the tint will be the exposure required.
To use this actinometer we must first get a standard exposure. Suppose that on a certain day we find that the paper takes two minutes to reach this tint, and that the correct exposure at f,`I6 is found to be one second. From these data we can write a memorandum as follows in the outside of the cartridge envelope.
Time of exposure per minute taken by the paper in reaching tint, given in seconds : . 3b sec.
• • • • • ji . .
• of . „ f/22 • . . I „ f/32 . . . . 2 „ This is a very rough-and-ready form of actinometer, but quite sufficient for ordinary practice. There are many excellent instruments of more convenient and durable form to be had at low prices, and they contain a very large amount of information helpful to the student. Actinometers
should not be exposed to direct sunlight, because detail in the shadows is the chief aim in most exposures ; just within the shadow of the operator himself will suffice.
Exposure Tables.—These must always be calculated for the latitude of the place, the variation at different times of the year not being constant. For instance, according to Spitaler's Tables the intensity of the light during the month of January at 40° and 50° latitude differs as 8o : 22 ; in April as 361 : 288 ; and in June only as 469 : 432. The observer should keep an exposure diary in which the time, nature of subject, and character of the light are carefully noted down for future guidance.
The following tables are only intended as a foundation for the reader to work upon. They represent the experience of some years, and are calculated for a plate registering about 125 on the Hurter and Driffield scale. The time given is the lowest at which detail in the shadow can be secured at f/8 when photographing a landscape the foreground of which is well lighted. (Latitude that of Southern England, or 51° 30'.) Except in dull wintry weather, half these exposures will often suffice for a passable negative. Subjects containing excessive contrast should have a good exposure ; those having few shadows and plenty of half-tones require con siderably less. The nearer the object is to the lens, the longer the exposure necessary to give it prominence.
Authorities differ as to the effect of east wind and rain upon the time of exposure. Some contend that a drying wind increases the intensity of the light, because it tends to remove the aqueous vapour which otherwise would intercept a portion of the rays ; others insist on a shorter exposure when the atmosphere is very moist. In practice we have found that with a moist atmosphere the foreground requires a much longer exposure : the background gains in illumina tion owing to the light reflected by the moist particles in the air. This applies to sea and lake pictures as well as to lucid intervals in wet weather.
Lighting.—Brilliant sunlight does not always produce the most artistic picture ; the gradations are softer and clearer in diffused light. It is rarely advisable to take a picture against the sun, i.e. with the lens pointing towards the sun in such a way that it has to be shaded to prevent the fogging of the plate. If the light is directly behind the operator, the shadows will tend to be tame and flat. Lighting at an angle is the best for the average picture. A very good general idea of the lighting of a scene, and of how it will look when the colouring is lost on a monotone silver print, will be got by examining it through a square of blue glass.
Maxims for for the shadows ; never mind the high lights. When in doubt, over-expose.