Woodbury's Photometer is perhaps the simplest constructed instrument of this kind. It was invented by the late W. B. Woodbury, and consists of a small, circular box having something of the appearance of a watch. A slip of paper passes under a hole in the centre of a disk printed with a scale of tints. The portion of the sensitized paper visible through the centre aperture being exposed to the light for one minute, it is carefully compared with one of the surrounding tints, whichever tint it corresponds with ; knowing the value of that tint, the necessary exposure is easily arrived at. If a bromide plate is used, bromide paper sensitized with a 10 per cent. solution of potassium nitrate must be placed in the photometer ; for a chloride plate chloride paper, and suitable tints, to be used as guides, prepared. This little instrument is also extensively used as a guide to the exposure of printing frames when making prints upon a carbon tissue, or by other processes where the action of the light is invisible. For illustration of Woodbury's Photometer, see Carbon Process.
Stanley's Actinometer is a similar arrangement, and consists of a ribbon of sensitive paper placed upon a reel, so that a small portion may be drawn out at a time under a glass cover, upon the under side of which is placed a piece of tinted paper. By taking the time required for the sensitive paper to arrive at the standard tint, the relative actinicity of the light is ascertained.
Decoudin's Photometer is an instrument for determining the necessary exposure by the apparent brightness of a portion of the image as seen on the ground glass. This is done by rotating a diaphragm pierced with various-sized holes, the point at which the illumination of a figure is just extinguished being marked with a letter which, on reference to a table, gives the necessary exposure.
Warnerke's this, phosphorescent paint is used. The light from the subject is allowed to fall upon it for one second. The paint then becomes luminous, and its brilliancy is estimated by viewing it through translucent screens varying in thickness.
Ballard's Actinometer is also based upon the absorption and luminous powers of phos phorescent paint Wall thus describes it.* "It consists of a square tube of wood having at one end a hinged cover, with, on the inside, a prepared surface and a spot in the centre. The method of using is to open the cover and look through the tube directly at the object to be photographed for half a minute, then closing the tube, a central blue spot will be seen which will gradually disappear. The number of seconds which it takes to do this, forms the basis of the calculation of a series of tables which accompany the actinometer." Both this arrangement and Warnerke's are unre liable, owing to the fact that the light emitted from the luminous paint is not constant ; further, heat has a marked effect upon the brilliancy.
Watkins Exposure will be seen in the accompanying diagram (Fig. i6), this little instrument consists of a brass cylinder with movable pointers and scales. At one end is a strip of bromide paper which is exposed to the light falling on the subject to be photo graphed. At the same time the other end of the cylinder is unfastened, and being on the end
of a chain, is set swinging, each swing representing half a second. The number of seconds it requires to darken the bromide paper to the depth indicated is thus cal culated ; one of the pointers is set to that number, and the other to the various factors, such as quality of subject (according to table), diaphragm and speed of plate. This done, the pointer E will indicate the exposure required.
Leutner's invention of Aug. Leutner, of Vienna. It consists of a metal basis firmly screwed to the printing frame. To this is fastened a small strip of felt, which presses the sensitive paper firmly against the scale. This scale consists of ten degrees produced by a photo chemical process on yellow plate glass. A movable indicator serves as an indicator of the scale we require to copy to. The change of the sensitive paper can be viewed from outside, and when it arrives at the tint corres ponding to the density of the paper, we know that the exposure is about enough. See Fig. 17.
The word sensitometer is often used to denote instruments of the kind described, but this is erroneous. Sensitometers will be found under their own heading, and differ from actinometers insomuch as an actinom eter is an instrument to measure the actinicity of light upon certain pho tographic preparations, and a sensitometer is an instrument arranged to measure the sensitiveness of such preparations to the actinic rays. See also Exposure Meters.
ACTINOMETRIC.—Pertaining to an actinometer. Under this head ing we will give the most interesting results that have (according to Professor Roscoe) been obtained by actinometers for measuring the chemical action of the solar rays. The diagrams of curves on the next page (Fig. is) show the variation of the chemical action of day light with the different months. Each curve is marked with the month, and denotes the rise and fall of the chemical intensity from 6 A.M. to 6 P.M. With this chart it will be seen that at mid-day in July the chemical inten sity is more than seven times greater than at mid-day in December. This chart was made at Kew, but it is a well-known fact that this chemical action of light varies to a very considerable extent in different portions of the globe. According to the various observations of photogiaphic travel ers we find that the nearer the equator the more difficult it is to obtain good photographs ; also the exposure in the tropics with the blazing sun is required to be much than on a gloomy day in England. Careful scientific observations have proved that the chemically active rays of the tropics are much greater than in this country. At Para, situated nearly under the equator, they were, according to observations made on a certain day by Professor Thorpe, seven times greater than on the same day at Kew ; therefore, if the statement of increased exposure be correct, we have to look for some other cause than the diminution of the sun's chemical intensity.