FACTORS REGULATING INTENSITY OF LIGHT.
Having chosen a suitable illuminant, it is necessary to have some measure of the intensity of the flame, as upon this de pends the length of time required for ek posing any particular brand of plate. Unless a determination of the actinic value of the flame be made there is no real guide to the exposures to be given, and consequently many poor or spoilt negatives may- be produced before a satisfactory picture is obtained. The light, after it has passed through the different lenses, is generally a very poor one, far below ordinary daylight in its power, therefore more latitude is allow able in the exposures than is possible in taking an ordinary photograph ; never theless it is much more satisfactory to know the value of the light than to time the exposures in a rule of thumb manner. The intensity of the light used will vary with the nature of the illuminant, the nature of the object to be photographed, the character of the objective, and other optical parts, and the distance of the plate from the source of illumination.
MEAstiumo INTENSITY OF LIGHT AND CONSEQUENT EXPOSURE OF PLATE.
The measurement of the intensity may be made directly by observation, or in directly by exposing a plate and develop ing. With either method it is necessary that the camera, microscope, lighting arrangement, and other apparatus should be placed in their proper positions, as will be subsequently described, and that separate measurements should be made with each objective and each eyepiece both alone and in combination, as the amount of light transmitted is naturally very largely affected by the lenses used.
A very simple method of direct measure ment is by means of a Warnerke's sensitometer screen ; this consists of a plate of glass covered with squares of coloured gelatine, upon each square being an opaque number indicating its value in the scale of opacities (Fig. 6S2). This form of screen was at one time used for de termining the speed of plates, but it seems now not to be obtainable. The amount of light passing through any square is nearly one-third more than that of the square which follows it. For the present purpose the screen is fitted in a wooden frame so that when it is placed against the ground glass screen no light, except that which passes through the sensitometer, is visible. The velvet cloth is drawn over the head, and a short time is allowed to elapse in order that the eye may become accus tomed to the dim light. A piece of card is now placed over the lower numbers if they show too brightly, and an observa tion is made ; the highest number of the sensitometer that is visible is read off, and the sensitiveness of the photographic plate, determined as already described, being known, the correct exposure is easily found. Several trial exposures have to be made in the first place, and a table of times showing the exposures necessary for light of various intensities is constructed for subsequent work.