MAKING AN EXPOSURE METER.
In order to construct an exposure meter, make a series of tints of a neutral or blue grey on a strip of card about 2 in. by in. by applying to the card repeated washes of colour, each successive wash being a little shorter than the preceding ; or the same end can be obtained by printing the tints from a graduated scale. Each suc cessive tint must be just a distinguishable shade darker than the preceding. Fit this card in a small frame fronted with a sheet of blue glass and backed with a stout piece of black paper in which is cut a slit A (Fig. 181) equal to the width of tint, and in length equal to the tint square, plus the width of the bromide paper B. Fig. 182 shows the order of filling the frame, c being the wood back ; D, the felt pad; F, the tint ; F, the black paper, and a, the blue glass. By this apparatus the ratio between the sensitive ness of the plate and the sensitiveness of the paper may be found, but for correct ratios it is essential that bromide paper, not printing-out paper, should be used. The action of a beam of light (or of the component parts of a beam) upon chloride of silver may be represented by the curve shown in Fig. 183, by which it will be seen that whilst considerable effect is produced by the blue and violet, very little reduction of silver is caused by the red, yellow, and green rays. On the other hand, the curve in Fig. 184 shows that bromide of silver, although most sensitive to blue, is more evenly sensitive, and by comparison of these curves (which are of course only rough) it will be seen that when the light is dull the meter will indicate ton long an exposure. On the other hand, as glass is partially opaque to the ultra violet rays, the ratio will be different in this regard also. Ordinary bromide paper shows little or no change under the action of light, but if washed over with a soluble nitrate it darkens rapidly. Take, for example, two pieces of bromide paper and rub one over with a solution of silver nitrate and expose them both to light, and the difference in action will be most apparent. Paper pre pared in this way may he employed for exposure in the meter just described.
How TO USE HOME-MADE EXPOSURE METER.
To find the ratio between the actual amount of light received by the object and that impinging on the plate, proceed as follows : Hold the prepared frame in the light that illuminates the object or scene to be photographed, and count the number of seconds that elapse before the paper exposed in the frame darkens sufficiently to match the different tints, pulling out a fresh square of paper for each tint. Now
photograph the scene or object, the correct exposure for which is to be ascertained by exposing the plate a bit at a time. This is done by drawing the slide one-tenth of the way out and giving one second of ex posure, and so on. When the last expos ure has been made the different portions of the plate will have had the following exposures—namely, 10 sec., 9 sec., 8 sec., 7 sec., 6 sec., 5 sac., 4 sec., 3 sec., 2 sec., 1 sec. That division which, on develop ment with normal solution, shows the best gradation is the division that is correctly exposed. The exposure needed to obtain the result desired, compared with the time taken to darken the paper to the selected tint, shows the proportions the two bear to each other. For instance, if at f.32 a dis tant view, well illuminated, required 1 sec. with plate speed 30, whilst the bromide paper in the same light needed 10 sec. to darken to tint 3, for such scene, and under such conditions of plate speed and r ratio the exposure for the plate in the camera is the tint exposure. (These figures must not be taken as actual proportions, but only as a guide. The proportion de pends on the tint, and must be found by experiment, as directed.) Suppose now that a similar subject, with a plate speed of 60 and IrS, gives sufficient depth and definition whilst the tint exposure is 60 sec., then as the increase of the plate speed will recluoe the exposure to and f'S to of that . : : 1 : 11P, and, as the plate exposure is of the tint exposure, 60 if T = the r (a x lu x time taken to darken, r = the ratio be tween actinometer and plate, a— the pro portionate plate speed, and b = the pro portionate f I value. Speaking roughly, this may be stated to give the correct exposure in seconds for, say, f/56. In the same light ascertain the length of time in seconds required-to tint the paper to either of the tints. This gives the pro portion of actinometer to plate. Say that the time required is sixteen times as long as the exposure, then the actino meter will ever afterwards, provided the same speed plates are used, indicate the exposure for f72:2, from which the exposure may be calculated with ease for any other stop.