Notes on Standard Wave lengths spectographs, and spectrum tubes . the grating space. The distance from the plate to the grating is R where R is the radius of curvature of the grating. This is thepolar equation of a parabola. One of our gratings having R = 640 cm and 7500 lines per inch(& = cm) permits the photography of spectra from Meggers! Spectroscopic Notes 195 2000 A in the first order when 1 = 5 degrees to about 6000 A in thefourth order when * = 45 degrees. The distance of the plate fromthe grating increases from cm in the first position to 374-9 cmin the last, so that t
Notes on Standard Wave lengths spectographs, and spectrum tubes . the grating space. The distance from the plate to the grating is R where R is the radius of curvature of the grating. This is thepolar equation of a parabola. One of our gratings having R = 640 cm and 7500 lines per inch(& = cm) permits the photography of spectra from Meggers! Spectroscopic Notes 195 2000 A in the first order when 1 = 5 degrees to about 6000 A in thefourth order when * = 45 degrees. The distance of the plate fromthe grating increases from cm in the first position to 374-9 cmin the last, so that the focal curve is not a circle, as in the case of a. Rowland mounting. This produces a gradual increase in the dis-persion with increasing wave length. At 3000 A 1 mm along theplate corresponds to A, at 9000 A it is A. Flat plates,8 inches (20 cm) in length record an interval of 2000 A in the first 196 Scientific Papers of the Bureau of Standards ] order spectrum, all of which is in satisfactory focus except theextreme ends. For wave length measurements relative to thesecondary standards the deviation from a normal spectrum iseasily corrected from a curve obtained by plotting the differencesbetween the actual values of the standard lines and those computedfrom the measurements on the assumption that the spectrum isstrictly normal. In addition to the stigmatic feature of this type of concavegrating mounting it has an advantage in occupying about one-fourth the space required for a Rowland or Abney can therefore be used in smaller laboratories and is less troubledby temperature gradients which are more like
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