Home >> Encyclopedic Dictionary Of Photography >> Acetic Acid to For Washed Emulsion >> Actinic_P1


drum, paper, piece, light, exposure, exposed and oxalate

Page: 1 2


ACTINICISM.—The chemical action of sunlight. See Actinic.

ACTINOQRAPI1.—An instrument invented by Mr. Hunt for regulating the variations of chemical influence on the solar rays ; fully described in the British Association Reports for The name has also been given to a more recent invention of Messrs. Hurter and Driffield for calculating photographic exposures. It consists of a small box, which can be easily carried in the pocket, and contains four suitably mounted logarithmic scales which correspond with the light, the lens, the speed of the plate, and the exposure. See Actinometer.

ACTINOLOOY.—The science of photo-chemistry.

ACTINOrIETER.—There are various forms of actinometers, which may briefly be termed instruments for measuring the actinicity of light. In photography, they are used in conjunction with certain logarithmic tables to endeavor to work out the necessary exposure to be given to the dry-plate. They may also be used for timing positive prints, especially those in which the image is invisible, such as in the carbon or asphalt process. The terms actinograph, photometer, and other fancy names have also been given to them. A description of the most notable kinds will be given.

Herschel's instrument devised by Sir John Herschel for measuring the intensity of the solar rays. It consists of a thermometer with a large bulb, filled with a dark blue fluid, and enclosed in a box, the sides of which are blackened and which is covered with glass. It is placed for a minute in the shade, then a minute in the sun, and then one more minute in the shade. The mean of the two variations in the shade is then subtracted from that in the sun, and the result measures the influence due to the solar rays.

Draper's Actinometer, invented by Mr. H. Draper, of New York, is based upon the fact that ferric oxalate, when exposed to the action of the light, gives out carbonic acid. This is explained by the following : Ferric Oxalate Ferrous oxalate Carbonic anhydride.

= The action of the light vibrations upon the ferric oxalate molecules is to split them up, and for each molecule so acted upon one molecule of carbonic anhydride is liberated. Draper's apparatus consists of a small glass cistern, containing exactly 50o grains of a standard solution of ferric oxalate. The cistern is covered over with an opaque covering, except one square

inch through which the light is admitted. After the exposure to the light, the amount of carbonic anhydride disengaged was calculated by the difference in weight before and after the exposure.

Roscoe's this the actincity of daylight or sunlight is measured by the exposure of sensitive silver chloride paper to its action for certain lengths of time at each hour. To effect this the piece of sensitized paper is fixed round a drum. This drum can turn on a horizontal axis, and the frame which supports it can slide horizontally. A piece of thin sheet brass is fixed over the drum, and a hole is cut through it. It is so arranged that it presses down lightly on the top of the drum ; thus a small piece of the sensitized paper is always exposed to the daylight, and the rest in total darkness. Thus, by the horizontal movement of the frame and the turning of the drum, every part of the sensitized paper will have been brought under the hole. The manner in which it works is this : The frame which supports the drum is con tinually moving slowly in a horizontal direction. Between the hours the drum does not turn, but only moves slightly in a horizontal direction. At the hour the drum turns suddenly for a small part of a revolution, thus exposing a fresh piece of paper under the hole. After two seconds it again turns suddenly as before, exposing that piece of paper for two seconds. In a similar manner fresh pieces of paper are exposed for 2, 4, 6, 10, 20, 40, 90 seconds, and after this the drum does not rotate again till the next hour, when a similar set of movements takes place. Hence, between the hours, there is a piece of paper exposed for 57 minutes, and which is useless on account of the long exposure. Owing to the horizontal movement of the drum, a fresh piece of paper is exposed when one complete revolution of the drum has taken place. An instrument of this kind is set up at South Kensington, and Captain Abney has designed a special reading-off apparatus which greatly facilitates the operations necessary for the daily readings.

Page: 1 2