On ultra-violet spectro-photometry

Abstract

In a previous communication (Merton and Nicholson, 'Phil. Trans.,’ A, vol. 217, p. 237 (1917)) the application of the neutral wedge to quantitative measurements of the relative intensities of spectrum lines has been described. More recently the method has been used by other investigators ( cf ., Plaskett, 'Dom. Astrophys. Obs. Pub.,’ 7, No. 12; Hughes and Lowe, 'Roy. Soc. Proc.,’ A, vol. 104, p. 480 (1923); Robertson, ‘Journ. Opt. Soc. America,’ vol. 7, No. 11, p. 983 (1923)), but its range has been limited by the fact that the wedges at present available are too opaque to the ultra-violet to be used effectively in that part of the spectrum. I have in an earlier paper described a method (‘Roy. Soc. Proc.,’ A, vol. 99, p. 78 (1921)) which is applicable to the ultra-violet, and which involves the use of a coarse diffraction grating which is crossed with the dispersing prism or grating of the spectrograph, the relative intensities of the different orders being calibrated; but this method suffers from the disadvantage that the intensity does not vary continuously, but changes in a definite ratio from order to order in a series of steps which are not very conveniently spaced.