The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Optical Centre
Ortol Developer
Optical Centre. An old term, the use of which is founded on a misapprehension. It was often defined as a point in the principal axis of a single lens which may be found as follows : - Draw a line to represent the principal axis, A c a' (fig. 101); then from the centres of curvature draw two radii, ab and a'b',
Fig. 101.
parallel to one another, but oblique to the central axis; join their extremities, b, b', and the point c at which the principal axis is cut by this line is the optical centre.
Optical Glass. See Glass.
Optical Lantern. See Magic Lantern.
Optics That
branch of science relating to the nature and laws of vision. The subject is too comprehensive to treat here. For further study the reader is referred to Glazebrook's " Physical Optics."
Orange Light. Actinic light filtered through any medium which absorbs all but the orange rays. It is frequently used for the illumination of the dark-room, and is sufficiently safe if not too brilliant.
Oroheliograph. See Panoramic Camera.
Orthochromatic. See Isochromatic.
Orthographic and Orthoscopic. Two fanciful titles given to certain classes of lenses.
Ortol Developer. See Development and Developers.