INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 141 



(III.) Th,e Method of Measurement. 



The method adopted for the determination of the intensities of lines in a spectrum 

 has been described in a previous communication,* in which it was shown that the 

 absolute values of the intensities can be obtained from the " photographic " intensities 

 by the adoption, as a standard, of the radiation from the positive crater of the carbon 

 arc, in which the distribution of intensity along the spectrum can be calculated by 

 PLANCK'S or WIEN'S formula. For the purpose of the present investigation, the 

 photographic intensities afford all the necessary information, and the results exhibited 

 below are accordingly limited to a determination of these values. 



The spectrograph consisted of a large single prism constant-deviation instrument 

 by Hilger, with a camera attachment in place of the telescope. Instead of the 

 V-shaped slide for reducing the length of the slit, a brass slide with a rectangular 

 opening was adopted, and in front of this opening was fixed the neutral glass wedge. 

 This consisted of a prism of neutral-tinted glass cemented to a similar prism of 

 colourless glass in such a manner that the combination formed a plane-parallel plate. 

 When light is allowed to fall on to the slit through this wedge, the resulting 

 spectrum is found to consist of lines which are bright at one end, corresponding to 

 the thin end of the wedge, and which fade away in the direction corresponding to 

 the dense end of the wedge, the length of the line on the plate thus depending on its 

 intensity and also on the " density " of the wedge for that particular wave-length. 



The spectra under investigation were photographed on Wratten Panchromatic 

 plates, and these were developed with a Hydroquinone and Formaline developer 

 which gives results showing great contrast. From the negatives thus obtained, 

 positives were printed by contact on Paget Half-tone or Paget Slow Lantern plates, 

 which were found to give the best results for this stage of the process. These 

 positives were then intensified with Mercuric Chloride and Ammonia, and enlarge- 

 ments were subsequently made on bromide paper using a Zeiss " Tessar" lens, which, 

 under the conditions of use, gave no measurable amount of distortion of the image. 

 The enlargements were made with the aid of a ruled process screen, which was placed 

 immediately in front of the bromide paper. The resulting enlarged negative image 

 was in this way built up from a number of small dots, one-hundredth of an inch apart. 

 On the enlargement obtained by this method, it is a matter of no difficulty to pick 

 out the last dot visible on each line, and thus to determine with considerable accuracy 

 the relative lengths of the lines composing the spectrum. In the absence of the 

 process of reproduction of the image in dots, this would be a matter of great 

 difficulty, and the results would be subject to considerable personal error. 



The plate-holder of the spectrograph was provided with a rack and pinion motion 

 in order to allow of the possibility of photographing a number of spectra on the same 

 plate. The spectra under comparison are thus photographed on adjacent portions of 



* Loc. cit. 



