introduce something often called astigmatism although widely, different from the astigmatism of a lens, by using a slot instead of a round *hole. Some of Mr. Dallmeyer's lenses, in which spherical aberration is corrected to the. utmost, are so mounted that the user can disturb the correction should he wish to do so. Hence it will be seen that aberrations, if properly understood, may occasionally be of service to the worker. (See the aber-rations under their respective headings: also Lens ; Pinhole ; and Monocle.)
Absolute. Pure or unmixed: often applied to alcohol quite free from water. (See Alcohol.)
Absolute Temperature and Absolute Zero. It has been assumed that at a temperature of - 2730 C, or - 4590 F., heat would be entirely absent and no further cooling would be possible. On this assumption temperatures reckoned from the absolute zero are called absolute temperatures. Thus 750 C, as ordinarily reckoned, would be 3480 C. on the absolute scale. (See Thermometer and Thermometry.)
Absorption. In optics, the partial destruction of light in passing through a medium: for example, a blue glass may absorb all rays but the blue rays, these latter being transmitted. Also the destruction of light by an opaque body, another form of energy (generally heat) being produced.
Accelerator. A term applied to any substance which is used to shorten the duration of development and to obtain the impres-sion of the slightest impact of actinic light. Thus, in alkaline pyro development the alkali is regarded as the accelerator, and with ferrous oxalate, hyposulphite of soda has been recom-mended. A few drops of a weak solution of hypo have a wonderful effect in bringing up detail in an instantaneous nega-tive ; indeed, the addition of a trace of hypo to ferrous oxalate developer sometimes materially shortens the time of exposure, but not infrequently it causes a partial reversal of the image. (See Development and Developers.)
Accessories. See Portraiture.
Acetates are salts formed by the reaction of metals or their oxides with acetic acid, the hydrogen of the acid being replaced