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BERKLEY AND MANSBERG
November
oscillograph tube. That is because observation with a particular camera is always made from exactly the same point thereby avoiding any errors caused by the parallax of an observer whose viewpoint may be shifting. Also, because of the geometry of the usually curved face of the cathode-ray tube, certain geometrical distortions occur, which may in some cases be eliminated or discounted in the photograph. There is also the possibility of so designing the cathode-ray tube as to eliminate this problem, resulting in orthogonal pictures from a curved cathode-ray-tube face.
Frequently, oscillograms may be enlarged and studied in detail to show characteristics which otherwise would be overlooked in a reduced or even in a one-to-one recording. For example, Fig. 2 shows the recording of a sine wave from an oscillator whose frequency was being modulated very slightly by another frequency, an undesired one. This defect is not visible ordinarily unless the image is enlarged to many times its normal size, in which case, it can be seen that the separation between these two cycles is different than that between the other two cycles.
One of the limitations of most cathode-ray oscillographs is that they have only a limited persistence time for the screen image. In the
Fig. 1 — Triply exposed oscillogram resulting from changes in circuit characteristics.
Fig. 2 — Continuous recording showing defects not visible on oscillograph.
case of very rapid transients, the persistence is too short to enable any retentivity for more than a few seconds. In the case of very long-time phenomena, the image at one edge of the screen may have disappeared at the time the image is being displayed on the other edge of the screen. By means of photography, we obtain what is virtually an infinite persistence. Photography may also be used to overcome one of