Notes on Standard Wave lengths spectographs, and spectrum tubes . h are mountedthe slit at S, concave mirror at M, concave grating at G and oamera at P are so close together, as seen from the mirror, that the aberrationdue to the mirror is small and the grating spectra are focussedin the camera P without appreciable astigmatism. This is anadvantage which permits the use of diaphragms in front of theslit for the photography of comparison spectra in juxtapositionand makes it possible to study the spectra of interferences or oflocalized parts of sources whose images are focussed on the
Notes on Standard Wave lengths spectographs, and spectrum tubes . h are mountedthe slit at S, concave mirror at M, concave grating at G and oamera at P are so close together, as seen from the mirror, that the aberrationdue to the mirror is small and the grating spectra are focussedin the camera P without appreciable astigmatism. This is anadvantage which permits the use of diaphragms in front of theslit for the photography of comparison spectra in juxtapositionand makes it possible to study the spectra of interferences or oflocalized parts of sources whose images are focussed on the are in reality two stigmatic foci, one for vertical lines Spectroscopic Notes 193 and the other for horizontal lines. In the case of our instrumentthe former is used for obtaining the narrowest slit images, and thelatter, which is about 2 mm farther from the grating, whensharpest definition along the length of the slit is required. Thespectrum, which is nearly normal, is formed on the photographicplate P, which is parallel to the face of the grating and may be. adjusted in its distance from G to put it in the focal plane. Thegrating and the camera are mounted on the steel beam B, which ispivoted on a vertical axis passing through the middle of thegrating. To pass from one region of the spectrum to another thebeam B is moved along the pier to definite positions, which arecalibrated for spectral region and camera focus. i94 Scientific Papers of the Bureau of Standards [vd a When the incident and diffracted beams make an angle i,the wave length X at the middle of the plate is b sin in in which n represents the order number of the spectrum and b
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