1044 



and L. Hamburger, does not allow to compare the intensities of the 

 radiations of different wavelengths. This can be done however by 

 determining the selective action of tiie optical arrangement and of 

 the photographic plate. 



For this purpose it proved in some definite cases desirable to 

 illuminate the sensitive plate, whilst using the rotating sector, with 

 known temperature radiators, which worlt at a very high temperature 

 and the energy-curve of which is known. The sensibilit}' curve arising 

 after the development (fig. J, plate I) was then compared with the 

 energy-curve following from the well-known laws of radiation. In 

 this way the change of sensibility of the plate with the wavelength 

 could he determined. 



Those who want to know more about the subject, must be referred 

 to my dissertation ^), where moreover a collection of objectively 

 reproduced emission spectra of a considerable number of gases and 

 their mixtures will be found, which at first sight give an im- 

 pression of the distribution of intensities, such at- least as has been 

 represented on the photographic plate. Such a collection was wanting 

 until now in the literature on the light emission by gases. 



§ 2. Measurements of intensitij for pure gases '). 



Investigations were made on the following gases: nitrogen, hydro- 

 gen, argon, neon, helium. As an example we shall first give a more 

 detailed discussion of the result found with nitrogen. We shall 

 however begin with referring to the spectrograms represented in 

 fig. 2. plate I. The first one is obtained by the action of light 

 emitted by nitrogen, when in front of the slit of the spectrograph 

 a wedge of smoked glass has been placed. The other one has been 

 made with the rotating sector. This reproduction is a good illustra- 

 tion of the very strong selective power of the wedge of smoked glass. 



A. Nitrogen. 



It may be allowed to mention in this place only the conclusions 

 we got from the observations on the sharp edges of the bands 

 of the second positive nitrogen group : 



1. The measurements can be reproduced. 2. The influence of the 

 time of illumination on the length of the edges of the bands is in 

 agreement with the calculations. 3. The emissive power of nitrogen 



1) L. Hamburger. Diss. Delft. 1917. In one chapter of this dissertation my 

 experiments in the phenomena shown by Testo-discharges in gases have been 

 described. 



^) For comparison with the measurements known in the literature see my diss. 



