A new prism arrangement of constant deviation type was devised, which is characterized by higher brightness, greater resolving power, and increased accuracy of wavelength as compared with types hitherto known. This arrangement consists of two equal half-prisms placed separately with their divided surfaces outward and a rotative plane mirror properly placed in some position between them. It is easily shown that the condition of minimum deviation is satisfied at every angular position of the mirror. When used as a monochromator, therefore, no errors occur in the wavelengths of emergent rays computed for the condition of minimum deviation, even when errors occur in the mounting of the prisms or the mirror. This was proven by tests on a quartz monochromator constructed on this principle. Further, the aperture of the instrument suffers no change by rotation of the mirror. Since the prisms are held fixed with their divided surfaces perpendicular to the axes of the collimator and the telescope, the aperture remains constant at its largest value available, quite independent of the wavelengths used. This results, naturally, in some gain of resolving power.
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