THE " INFALLIBLE" EXPOSURE METER.
Another good form of meter is the " In fallible," introduced by Geo. F. Wynne, which selects from the numerous factors governing the exposure three only—light, plate sensitiveness, and stop. This instru ment is founded upon the same principle as the. Watkins exposure meter, the intensity being gauged by the time taken to darken to a standard tint. Two tints are provided, the darker one for general work and the lighter one for views under trees, interiors. or other dimly lighted subjects. Fig. 179 opening, it is only necessary to revolve the back half of the watch-case, when the sensitive paper will turn with it. Revolving the dial removes the orange glass, and leaves the paper ready for exposure. The actinometer time having been taken by carefully noting the number of seconds taken to darken to the tints as before de scribed, the disc is revolved until the plate speed is against the actinometer number ; then against every stop will be found the correct exposure, so that whichever is used the exposure will be known without calculation. Should the sensitive paper take minutes instead of seconds to darken to the standard tint, the actinometer time must be read as minutes, and the answer will be in minutes or fractions of minutes.
A set of plate speeds are issued by Mr. Wynne, in a book sold with the meter.
shows one of these instruments. It is cer tainly of a very convenient shape, being in the form of a watch. The dial is in two parts, the outer ring bearing the f numbers or plate speeds, and the inner circle the actinometer time. At the top is a V shaped opening, on each side of which are the two tints. This opening is covered when out of use by a piece of yellow glass cemented outside the glass dial. The dial is made to revolve so that any f number can be brought opposite any actinometer num ber. The back opens, and discloses a disc of prepared bromide paper backed by a piece of felt and a wire spring. Thus, to bring a fresh piece of paper before the Suppose now the actinometer takes 48 seconds to darken to the deepest tint, and the plate speed f20 is being used, then the dial will be revolved as shown in Fig. 180, when it will be seen that the exposure at /16 is 32, and so on. Pinhole exposures are calculated in the same manner by dividing the diameter of the hole into the extension.