EXPOSURE METERS.
This has brought about the introduction of exposure meters-, which consist of an arrangement of figures that by a revolving or other device may be brought into differ. ent relation to each other, the figures being arranged under different headings, namely, the factors governing exposure. Perhaps one of the simplest among these is the Ilford exposure meter. It consists of a series of three revolving discs as shown in Fig. 176. These indicate (1) plate, (2) diaphragm, (3) date and hour, (4) sub ject, and (5) required exposure, the first and last being fixed. To use it, the arrow on the date circle is placed on the circle opposite the plate to be used. Next, the arrow on the diaphragm circle is placed opposite the date and hour, and the arrow on the subject circle opposite the stop to be used. The necessary exposure in each case matching the darkening of the paper, either the actinometer time or the exposure when found must be multiplied by the denomina tor of the fraction which they form of the actinometer time. For example, if a half tint is used, the exposure will be double that directly indicated, and if a quarter tint is used, four times that. As to whether the actinometer time or the actual time taken to darken the tint is multiplied by this number, or whether the exposure indicated by the instrument is so multi plied, is of no importance. The instrument
described above is the will be shown opposite the subject. In other instruments means are provided for exposing a piece of the specially pre pared bromide paper referred to above. By the side of the portion exposed are various tints through which the paper passes before reaching its deepest slate blue—one tint shows the darkest to which the paper will go, another tint shows the shade reached in half that time, a third that which it reaches in a quarter of the time, and, if desired, even lighter tints. The end revolving ring shows in one i;et a series of numbers representing the relative speeds of different makes of plates ; another set shows the stop numbers from f4 up to f64 ; a third set gives a series of numbeys representing in seconds or minutes the length of time the prepared bromide paper has taken to darken to the deepest tint. Another set shows the pro portionate exposure required for different subjects ; and the last set shows the correct exposure. If one of the lighter tints, such as a half-tint or a quarter-tint, is used in