This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Securely fasten a sheet of newspaper to a perfectly flat surface, and then sharply focus the camera in the center. If upon examining the sides of the paper the letters are indistinct and blurred, the lens has not been corrected for spherical aberration.
A lens in which the various combinations or cells may be used separately, thus giving different focal lengths. A triple convertible lens is one in which it is possible to have three different focal lengths. (See Lens, Unsymmetrical.)
A lens constructed of three separate combinations of glasses.
A rectilinear lens in which the two combinations have different focal lengths. One of the single lenses will give an image about double that obtainable with both cells in combination, while the other cell will give an image 1/3 larger than the former cell.
A lens embracing a wide angle of view. (See Angle of View.)
A small instrument used to determine whether the base of the camera is perfectly horizontal.
A ray of white light is decomposed on passing through a prism, i. e., separated into its constituent color rays.
An optical term applied to rays of light converging or diverging from a point.
A non-actinic source of light for illuminating the dark-room. When made of proper material this light can be used with safety for handling and developing all ordinary dry plates. But, trichromatic and panchromatic plates must be handled either in total darkness, or a specially prepared green safe light.
Ca(ClO)2 + CaCl2 + 2H2O. Bleaching Powder or Lime Chlorinated. Deliquescent white powder; unpleasant saline taste. Partly soluble in water. A bleaching and disinfecting agent. Used as a hypo eliminator; also in toning baths.
Light produced by a jet of combined oxygen and hydrogen gas, being thrown upon a cylinder of lime, producing a dazzling white light.