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light, plate, holes, plates, scale, time and sensitive


SENSITOMETER.—An instrument for comparing the sensibility of various photographic sensitive surfaces. There are various forms, which are, however, all more or less imperfect. The reason of this is that a standard light is required, perfectly uniform, and reproducible at any desired place or time. A light of this description has never yet been found, nor has any means yet been devised of calculating the difference between an accepted standard and an actual light made as near to it as possible.

Warnerke's Sensitometer is the one generally employed in testing various photographic sensi tive surfaces. It consists of a transparent scale of figures made by the Woodbury process, the transparency of the scale decreasing by regular degrees from i to 24. Under this scale the plate to be tested is exposed for a given length of time. Warnerke employs what he terms a normal light, that is, a glass pane coated with a phosphorescent paint. Before this, two-and-a-half centi meters of magnesium wire are burned, then the whole is left undisturbed a minute, when it is placed upon the scale under which the dry plate lies. This light is considered as always uniform, but this is not so, however, as the temperature has a very marked effect upon the luminosity of the phosphorescent paint, the mere warmth of the hand increasing it. Further, this instrument possesses the drawback of being unable to register small differences.

Taylor's Sensitometer.---In this a more perfect method of graduating the light that reaches different parts of the plate from a uniform source is employed by means of a series of vary ing areas of a uniformly illuminated surface. The apparatus first employed by Arthur Taylor, in 1869, consists of a box containing a series of short wide tubes, each of which was open below to the sensitive surface, and closed at the upper end with a diaphragm perforated with a certain number of exactly uniform holes. The intensity of the light acting upon the sensitive surface at the bottom of any tube is proportional to the number of holes in the cover at the upper end of the tube.

Vogel's ,Sensitometer is an improvement upon the previous one. The scale is formed by a metal plate with holes. This plate covers a wooden block in which twenty-four cylin drical cells are drilled. Above the first cell one hole is made, above the second two, above

the third three, and so on. Under these cells, the plate to be tested is exposed in a dark slide-like contrivance, and it is obvious that the relative clearness under the different cells must be in exact proportion to the number of apertures made above the respective cells. When, therefore, two plates are exposed equally long under this instrument before an object uniformly illuminated and then developed, it may be that one plate will show the effect of light up to cell number two (with two holes), and the other up to cell number four (with four holes). With half the strength of the light the same result was obtained with the first plate as with the whole strength in the second plate, and the first plate is therefore shown to be twice as sensitive as the second.

Mucklow and Spurge's Sensitometer.---This is also similar to Taylor's, except that the dia phragms have one opening each, the openings being made of the required sizes. It is obvious that when a developable film is being tested the light must be graduated, because a uni form effect cannot be secured (the time then being in inverse ratio to the sensitiveness), as the result is not apparent until development.

SENSITIVE.—Capable of undergoing change by exposure to light.

SENSITIVENESS.—The sensitiveness of photographic substances, films, plates, paper, etc., is determined by the amount of time required for a given quantity of light to impress itself upon them either visibly or rendered visible by after development. The sensitiveness of dry plates is usually calculated by means of Warnerke's sensitometer. This method is, however, far from being perfect, and although plates are often sold with the sensitometer number marked upon the box containing them, yet this cannot be taken as a sure guide to their rapidity of action, and the numbers are often more misleading than otherwise.

The sensitiveness of dry plates can be considerably increased by immersing them for from three to five minutes in a solution of too parts of alcohol •8o5, one to two parts of a i in 15 solu tion of nitrate of silver, and ten parts of ammonia. They are then dried and used immediately.