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PRINCIPLES OF CINEMATOGRAPHY
of the viewer. In some cases unusual physical reactions have been experienced by people viewing alternate red and green pictures, first by the left eye and then by the right, and this system did not meet with any great success.
The actual separation between the two camera lenses is not always kept at the normal separation between the eyes. When the cameras are focused on a very long shot it is normally necessary to increase the separation from 2-5-inches (6-35-cms) up to as much as 4-inches (10 16-cms) and, when focused down to a big close-up, the separation is usually decreased to the order of 1-5-inches (38-10-cms). Camera lenses used for this work are usually specially selected to ensure that they are perfectly matched and are usually so mounted that the separation between the two viewing points may be quickly adjusted by a mechanism connected with the normal focusing mount.
The view-finder used with stereoscopic cameras should also be of the double-ocular type, so that each lens system is matched with the field covered by the corresponding objective and also to ensure that each image may be adjusted to correspond exactly with the field covered by the respective objective lens.
0) Stereoscopic Pictures by Polarised Light. The unpopular use of filtered beams was then overcome by isolating the two pictures by polarised light which will, of course, maintain the normal black-andwhite images obtained with two-dimensional photography. Light waves may be considered to radiate in all directions and may be conveniently visualised from Diagram 'A', Figure 181, in which the two principal radiations, at right-angles to each other, are shown. Light is said to be
Fig. 181. — The polarisation of light waves.