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BERKLEY AND FELDT
July
drying process can be seen. This method may also be reversed by moving the phototube rapidly around the sample and drawing a great many curves as the sample dries.
POLAR PRESENTATIONS
It is more conventional in optics to express these light-distribution patterns in the form of polar rather than Cartesian co-ordinates. These patterns are generally used to describe the light distribution from lighting equipment such as photographic flood lamps. These patterns may be produced readily on cathode-ray oscillographs by
simple means. Fig. 5 shows an instrument for this purpose, the Type 275 polar co-ordinate indicator. This instrument produces polar diagrams which are synchronized with the motion of any rotating object merely by attaching a small lightweight, two-phase generator to the rotating part.11 If a phototube is placed on the rotating shaft its output can serve as a measure of the amount of light reaching a certain point from all angles by applying the output of the phototube to the radial-input Fig. 5-Polar co-ordinate oscillograph, terminal on this indicator. Similarly, the light source may be
rotated and the phototube remain stationary. Fig. 6 (a) shows a Cartesian light distribution obtained from small flashlight bulb. Fig. 6 (b) shows a similar light-distribution pattern in polar coordinates on the Type 275-A indicator. Phototubes and cathoderay oscillographs have also been used to scan the illumination of scenes being photographed to depict graphically the variation between high-light and shadow brightness so that, for example, the contrast range of the recording film is not exceeded.12
AUTOMATIC H AND D CURVE PLOTTING
One of the best specifications for the behavior of a photographic emulsion is its H and D curve which relates the photographic density