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camera. In stella photometry the photo-tube is accurate in the measurement of distant stars brilliancies. In lamp photometry it gives a nearly instantaneous means of measuring light brilliancies of a room or a moving picture screen, or of a light source. In some cases the use of color filters, which are composed of specially tinted glasses, is an advantage to eliminate undesirable light rays, which if permitted to remain would confuse the issue.
In again reverting to the first classification (A) in which the light ray passes through the subject, we have in a special case where a means of density control is necessitated by a photo-tube. A photo-tube of a special nature is required, this phototube was evolved because of the necessity for a tube of linear or straight line characteristics and stability which was unattainable in the present day form of manufactured tube. The sensitive area in this type of tube is composed of (if I remember
correctly) nickel-chrome wire coiled in a form of a cone (Fig. 6). Two such cones being joined at their apexes, a small voltage is then applied which causes them to glow faintly (about a dull red) . This tube is one of the “actino” family, as it is resistive in action. At the moment this tube is being used in reclamation of silver from “hypo” in a large film laboratory. At a particular point in the equipment used for reclamation, a photo-tube is needed to determine the specific density of the hypo-solution within extremely narrow limits of “density,” which necessitates a photo-tube of these characteristics.
The use of photo-tube in the reading of light vibrations or fluctuations in conjunction with an oscilliscope, gives an accurate means of studying the wave form of any unknown light source of either near or distant origin.
A photo-tube, when used in conjunction with an oscilliscope, is connected in such a manner as to pick
CIRCUIT CONTROLLING GALVANOMETER
FIG. 5
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