A device is described which automatically combs a list of wave numbers punched on a paper tape for pairs of lines having constant differences within a given tolerance, and records the wave numbers of these pairs on motion picture film. It supplements the spectrum interval sorter previously described, which makes use of the same punched tape and automatically places all similar intervals in juxtaposition so that frequently recurring intervals can be detected. The interval recorder is set successively for the various intervals which the interval sorter chart shows to be important. When a light beam passes simultaneously through holes in both upper and lower sections of a looped portion of the tape, a detecting photo-cell flashes an argon lamp which momentarily illuminates two wave number dials turning in synchronism with the motion of the tape, and records their readings. By means of a calibrated differential the two dials and the tape can be set for any desired wave number difference. By controlling the width of the scanning beam the tolerance for recording can be set equal to the interval variation which it is desired to include. With the aid of this machine and the interval sorter some 2000 lines in the spectrum of Ce II have been classified in a quadratic array by Albertson and the writer. The average difference between observed and calculated intervals, with wave-length measurements made on the automatic computing comparator described by the writer, is slightly over 0.01 cm−1.
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