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• W2031437360 abstract "The use of the neutral wedge for the determination of the relative intensities of spectrum lines has been investigated by J. W. Nicholson and the writer,* and similar methods have been used by a number of other investigators. The wedge consists of a neutral glass wedge cemented to a similar wedge of clear glass so as to form a parallel plate, which is mounted immediately in front of the slit of the spectrograph, or a Goldberg wedge, which is made of gelatine containing carbon in a fine state of division, may be used. With this arrangement a photograph of a spectrum shows lines which are black at one end, corresponding to the thin edge of the wedge, and which fade away with increasing density of the wedge. The photographic intensities are determined from the lengths of the lines. The density of the wedge at any point is defined as log 10 (I O /I), where I O and I, respectively, are the intensities of the light incident on the wedge at that point and of the light after passing through the wedge. We require to know the density gradient of the wedge as a function of the wave-length. If the density gradient D λ be the change in density per millimetre on the wedge at wave-length λ, and h 1 and h 2 the lengths of the lines on two photographs (taken, for example, under different conditions of excitation), the intensities will be proportional to log 10 -1 (h 1 )D λ / m ) and log 10 -1 (h 2 D λ m ) where m is the magnification of the slit on the photographic plate. Methods of determining the density gradient of the wedge as a function of the wave-length have been described in previous communications ( loc. cit .). The relative intensities of lines of different wave-length can be measured by comparison with a source of light giving a continuous spectrum, in which the distribution of intensity is known. For this purpose a black body at a known temperature would be ideal, but its use is attended with considerable experimental difficulties, and in the investigation referred to ( loc . cit .) the positive crater of the carbon arc was used as a standard source and was assumed to give the distribution of intensity appropriate to a black body at a temperature of 3760 K. Let us suppose that two photographs are taken through the wedge, one of a discontinuous spectrum under investigation and one of the continuous spectrum of the carbon arc. Considering now two lines in the discontinuous spectrum 'λ 1 and λ 2 , a measurement is made of the lengths h 1 h. 2 of these two lines on the plate, and also the heights h 3 h 4 of the blackened image given by the continuous spectrum at these wave-lengths. Let the relative intensities in the continuous spectrum at λ1 and λ2 (calculated from Planck’s law on the above assumptions) be c 1 and c 2 . It has been shown ( loc . cit .) that the relative intensities affecting the plate in the case of the continuous spectrum at λ 1 and λ 2 will be c 1 (λ 1 — λ 0 ) 2 and c 2 (λ 2 — λ 0 ) 2 , -where λ 0 is the constant in the Hartmann formula λ = λ 0 C /(n + n o ), which relates the wave-lengths and the measured positions of the lines on the photographic plate. Defining the photographic intensity pλ as log 10 -1 (h λ )D λ /m), the true relative intensities of the two lines are proportional to c 1 p 1 (λ 1 — λ 0 ) 2 / p 1 are and c 2 p 2 (λ 2 — λ 0 ) 2 / p 2 are . This method is independent of the relation between the density of the image on a photographic plate and the various conditions which affect it, and involves only a measurement of the lengths of the lines* which can, by suitable photographic technique, be made with considerable accuracy on the original photographic plate without recourse to processes of successive copying to increase the contrast. This technique, however, involves very considerable exposures, which may, indeed, in the case of faint lines become prohibitively long, and the measurement of the lengths of the lines is necessarily a somewhat laborious process, and is particularly troublesome in the case of spectra which are so complex that the identification of lines cannot be carried out by inspection, but must depend on micrometric measurement. It is believed that the method described in the present communication is a considerable improvement on the procedure described above, and is particularly suitable in the investigation of faint and complex spectra. In recent years a number of instruments have been designed for the determination of the densities of images on photographic plates,f and these instruments, with which a very high degree of precision is attained, have been applied to the determination of the relative intensities of spectrum lines. It is not proposed to discuss these methods in the present communication, but it must be stated that the procedure of deducing the intensities of the lines from the densities on the photographic plate is complicated and indeed precarious, since two sources of light of the same wave-length can only beconsidered to be of the same intensity when they produce the same density on the same photographic plate with the same time of exposure. The difficulties involved have been discussed by Toy,* who emphasises the desirability of using the blackening of the photographic plate as a null method only. The method here described does not attain to the accuracy of these instruments in so far as density measurements are concerned, but it enables the relative intensity of a line to be determined directly in terms of a standard continuous source of radiation. The spectrum under investigation is photographed without the interposition of the wedge or any other device, the exposure being thereby shortened, the only restriction being that a specified slit-width must be used. This slit-width is slightly greater than would be used under ordinary circumstances, but not so great as to diminish the purity of the spectrum to any serious extent. The standard continuous spectrum, e. g ., that of the positive crater of a carbon arc or of a standardised incandescent lamp, is photographed on an adjacent portion of the same plate, and for the same exposure time through a wedge system, the width of slit being set at such a value that with this exposure an image of appropriate density is obtained. In place of the wedge used in previous investigations the following arrangement has been adopted. A plate of very thin glass was coated with a black medium and ruled on a dividing engine with lines 0·2 mm. apart and 0·03 mm. wide. It is essential that the medium should be completely opaque, and the ruled lines completely transparent and of uniform width. A good deal of experiment was required to attain this result, and deposits of carbon, metallic deposits of silver and platinum, and a large number of varnishes were tried. Rulings through metallic films were either too transparent or else invariably showed a serrated edge when examined under a high power. A varnish known as “ Cellire (Black),”f which is a quick-drying cellulose varnish, was the only medium which gave satisfactory results. This medium is poured on to the plate, and the excess is quickly shaken off, the plate being then set aside in an inclined position to dry. The time of drying depends, of course, on the temperature, but at a certain stage the surface can be ruled through with a hard steel tool so as to give perfectly clear and uniform lines, which show no trace of irregularity." @default.
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• W2031437360 date "1927-01-01" @default.
• W2031437360 modified "2023-09-26" @default.
• W2031437360 title "On measurement of the intensity of spectrum lines" @default.
• W2031437360 doi "https://doi.org/10.1098/rspa.1927.0017" @default.
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