A Seya-Namioka type vacuum spectrophotometer incorporated with a concave grating of 1200 lines/mm and 1m radius has been constructed.
In consideration of the discussion given by Greiner and Shäffer, the spectrum of positive 1st order with the angle of 69°45' between the incident and exit light beams is used instead the negative 1st order spectrum with the angle of 70°15' used by Seya and Namioka.
The grating is rotated simply about its vertical axis through the grating center, and good focus is obtained over a wide range of wavelength from 0 to 3500Å, but actually the spectrometer is available to longer wavelength of about 6200Å, the limit of the response of RCA 5819 or 1P21 photo-multiplier.
The spectral lines can be resolved down to 1Å and the relative intensity of scattered light does not exceed 2 percent in the extreme ultraviolet region.
The spectrometer is designed to be used also as a spectrograph and may easily be modified into a 3m radius grating spectrometer if necessary.
As the light source for photometry in the extreme ultraviolet region, hydrogen discharge of 1 ampere at about 1mm Hg is used. Obtained spectra of light sources are shown and results of absorption measurements on LiF single crystals are given.
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