A Manual Of Photography

THE FOCIMETEB. 159 please. Fig. 22 shows the result obtained on a plate by this instrument; the letters corresponding with the holes in the other woodcuts. In Fig 23 the screen with the vertical slits is shewn at the moment it is supposed to be passing over the holes A B c D. In this example the plate had been exposed to the vapour of iodine, in such a manner that one zone had attained the first coating of yellow colour ; a second zone had reached the red ; a third the blue and green ; and a fourtli having passed through all these tints, had obtained the second yellow coating. The number of white circular spots on each vertical zone indicates the degree of sensitiveness of the various coatings; the less sensitive being the first coating of yellow, p, and the most sensitive the second yellow coating, a." This is shewn by the deposit of mercury on the plate represented by the increased whiteness of the spots corresponding with the holes, eacli four vertical spots having been exposed for the same time to solar influence. THE FOCTMETEB. 24. M. Claudet has also devised a very ingenious instrument for focusing, whicli he calls his Focimeter. (Fig. 24.) This it will be seen from the accompanying woodcut consists merely of segments of a circle, numbered and placed at fixed distances apart, upon a moveable axis. This is copied by the camera on u a plate or paper, and the result is shewn in the annexed figure ^ (25), in which it will be seen different degrees of eflect are sup-1 posed to have been produced. These determine the best focal point for any lens very readily, and it is really a most useful piece of apparatus in the hands of the photographer.