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Originally appearing in Volume V21, Page 518 of the 1911 Encyclopedia Britannica.
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SPEEDI INDEX-titloolltnnnullllIIIIEIII•IIIIIIIIIIIIIIIIIIII\IIIIBiinIll11I11\~\I1111111t1 300 20' 100 60.6050.0 30 20 09626 5. 3 2 100 003 SPIED OF PLATE ',HURTER IDRIPiI EID'S ACTINOGRAPI2 . PAtENT NO 5545 1888. forms based upon an actinometrical test of the light at the time of exposure. In the complete " Standard Meter " (1890) scales corresponding to " speed of plate," " diaphragm f numbers," " light," " subject " and " enlarging," marked P. D. A... and E., are arranged on rings adjustable round a cylinder. The plate-speeds are taken from a table and the " light coefficient," or " actinometer number," is ascertained at the time by ex-posing a piece of sensitive paper In the actinometer at the end of the instrument for the number of seconds required to match a fixed tint as shown by an attached pendulum. Many improvements have been made in it and the latest pattern (1908) is made in magnalium (fig. 69). The " Dial " meter (1901) Is a simpler form in a circular metal case with four FIG. 69.—Watkins's "Stan-apertures marked " plate," dard " Meter. " stop," " act " and " exp." above the corresponding scales, and an actinometer for testing the light. The numbers showing the speed of the plate in use, the f value of the diaphragm, and the actinometer exposure in seconds are brought into the respective apertures and the exposure required is read off in the exposure " aperture. An " indoor meter " is also made, and a " hand camera calculator " for use with the " Standard or " Bee " meters. The " Queen Bee " and " Bee " meters (1903) are later, smaller and more convenient patterns which have superseded the " Dial " meter and have the plate numbers and exposures marked round the case, and the scales of " f numbers " and " light " on a revolving glass plate. This is revolved till the f number on the right is opposite the speed number of the plate; opposite the "actinometer number " on the left, found as above, will be found the exposure in seconds (fig. 70). The " Queen Bee " meter is similar to the " Bee," but of better construction and fitted with a pendulum. G. F. Wynne's " Infallible " exposure meter (1893) is also in dial form, but the sensitive paper is exposed directly, no pendulum is used, and the scales are open on the dial. In use, the glass carrying the movable scale is turned until the actinometer time in seconds upon the exposure scale is opposite the diaphragm number of the plate, as given in the list of plate speeds; the correct exposure will then be found against each stop given on the scale. There are practically only two scales: the scale of dia- phragms representing the dia- phragm apertures or f numbers, the speed of plate and the vari- ation of exposure due to subject; and the time scale, represent- represent- 'htitid th FIG 7.Wynne's " If al-g te acnomeer tme ane li.ble1"—Exposure Metern. exposure (fig. 71). The actino- meter is protected by a yellow glass screen when not in use. In a smaller form the scales are on the OUIt\\111111111111111111111\OIIIIIIII\111111\1111111\~ IIIIIIIIII~~1111 2 3 23 1231212 121231212312123121231212 1EN4 26 • 55 8 10 113 w 6 20 226 28 32 b 412 51 6. 05002oP1 ~i E00 '505 2 3 • 56E6110 A tl 50 " Bee " Meter. circumference of a locket, and the actinometer at the back. An " Infallible " Printmeter is also made for showing exposures in contact printing on sensitive papers, but can also be used for testing speeds of plates and papers. Beck's " Zambex " Exposure Meter gives the exposure and stop to be used, also the depth of focus to be obtained with different diaphragm apertures. The required exposure is set to the " speed " number on the next scale of the meter. The third scale corresponds to the times of darkening the sensitive paper in the actinometer attached to the meter, and shows the diaphragm aperture suitable for the given exposure. Other scales show the distances that will be in focus with the different stops used, arranged so that the focal depth of four different lenses can be found. Several other exposure meters are made on the principle of the slide rule, with scale corresponding to the factors of " plate speed," " diaphragm number," " light," " subject," " exposure," and the exposure is found by simple inspection without an actinometer. They are designed for use with particular brands of plates, but can be used for others of similar speeds. Another class of exposure meters comprises those in which the intensity of the light is estimated visually by extinction through a semi-transparent medium of increasing intensity, such as J. Decoudun's (1888), in which the exposure is judged by the disappearance of a series of small clear openings on a graduated scale of densities when laid on the most important part of the image as seen on the ground-glass. Its indications are not very definite, and the paper scale changes in density after a time. A better form is " E. Degen's Normal Photometer " (1903), consisting of two sliding violet glass prisms, one adjusted for the diaphragm apertures, the other for the actinic illumination of the object. They are mounted with their outer faces parallel. In use the upper slide with prism is drawn out so that the pointer coincides with the division indicating the diaphragm aperture to be used; the object to be photographed is then viewed directly through openings at one end of the instrument, and the lower slide is drawn out and pushed back slowly till the object viewed is almost obscured. The attached pointer will then indicate the exposure required, or, reversing the order, the diaphragm aperture for a given exposure can be found. Auxiliary scales are attached for very short or very long exposures. The pnnciple of construction is that the logarithms of the times of exposure are proportional to the thickness of the coloured prisms. ' G. Heyde's Actino-Photometer " (1906) is on a somewhat similar principle, and consists of a circular metal box with dark violet glass viewing screens in the centre of both sides, with an obscuring iris inside the case worked by revolving the back of the box. On the front of the instrument exposure tables are given for plates of every rapidity, and for diaphragm apertures from f/3 to f/43. Exposure meters of this type are specially applicable for open-air work where there is sufficient light for ready measurement. Other simple actinometers are in use for carbon and process printing, consisting generally of trans-lucent graduated scales in different densities of paper, coloured gelatin, &c., or of a photographed scale graduated by increasing exposures. The " Burton actinometer," for pigment printing, made on this principle, contains several small negatives of different densities, one of which is selected of equal depth to the one to be printed, and the progress of the printing is estimated by exposing a piece of sensitive paper under it and examining it from time to time.
End of Article: SPEEDI
JOHN SPEED (1552–1629)

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