A device is described which can be attached to any standard moving-plate comparator which automatically records the passage of the density maximum of a spectrum line, computes the wavelength corresponding to this maximum and records this value to eight figures photographically at high speed. The automatic recording, computing and measuring features can be used independently if desired. The comparator screw can be driven by hand, or by electric motor at controllable speed, through a shaft which is coupled to a wavelength shaft by means of variable ratio gears and a differential so that the speed ratio of the shafts can be rapidly adjusted to any desired instantaneous value. When measuring a plate the operator sees on a screen before him a magnified image of a portion of the spectrum; the light in this image actuates an amplifier system through three photo-cells to give records of plate density and rate of change of density with distance. When the density slope becomes zero while the density is greater than some predetermined background value a mercury arc flashes to record the wavelength dial readings. Correct wavelength readings for any desired number of standard lines can be set into the machine, or an empirical dispersion formula can be introduced. Errors due to backlash, oil film variation and temperature variation of the screw are practically eliminated, and automatic setting on most lines is found to be more accurate than hand setting. A microphotometer trace is superposed by the machine on the wavelength list so that the intensities and physical characteristics of lines can be determined directly. A ten to 200-fold gain in speed of measurement of complex spectrograms results from use of the machine. When used as a microphotometer the instrument is faster than those in common use.
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