On spectrophotometry in the visible and ultra-violet spectrum

Abstract

In a previous communication the use of the neutral wedge for the determination both of the photographic and absolute intensities of spectrum lines has been discussed, and the results of its application to the study of certain phenomena relating to the spectra of hydrogen and helium have been given. The method has been found to be simple and convenient for the study of the relative intensities and of the structure of broadened lines in the visible spectrum. There are, however, certain features of the method which under some circumstances limit its application; in particular the density of the “neutral” wedge increases with the wave-number, and for the investigation of the ultra-violet down to λ = 2000 A, the method is at present inapplicable. Although it might be possible to construct wedges of crown glass with a very small angle for use in the ultra-violet, the fact that a number of such wedges would be required for different ranges would destroy the principal advantage of the method. Whilst no special precautions are required in the investigation of extended sources of light, it is evident that when small sources of light are used it is of the utmost importance to ensure that that portion of the slit which is behind the wedge is uniformly illuminated before the wedge is put into position, and in the case of selected regions of a source of light, e. g. , the spectrum from a particular point in the electric arc, it is only possible to obtain correct results by means of devices which entail a very considerable loss of light. The present communication relates to a method which, whilst not superior to the wedge method in accuracy, has the advantage that it can be used in any part of the spectrum which can be photographed through quartz lenses and prisms, and its application to the extreme ultra-violet beyond λ = 2000 A should present no serious difficulty. The method consists in crossing the dispersing system, e. g. , the prism of the spectrograph, with a very coarse grating, and reducing the length of the slit to a very small value. The grating is inserted between the prism and the camera lens of the spectrograph with the rulings perpendicular to the refracting edge of the prism, and a continuous spectrum thus appears on a plate as a dark central strip with a succession of other strips of different intensities on either side, the intensities of these orders being determined by the ruling of the grating and the width of the strips by the length of the slit. In the case of a discontinuous spectrum the "lines” are found to consist of dots of different intensities on either side of the central dot. It is evident that if the last dots which are just visible in the case of two lines are noted, a previous knowledge of the relative intensities of the different orders corresponding to these dots at once enables the relative intensities of the lines to be determined. It will be noted that the fact that the slit is reduced to a point not only enables different regions of a light source to be investigated without difficulty, but suggests a further application, which will be considered later, in the study of stellar spectra.