Physics. — ''On a method of determini7ig spectral intensities by 

 means of photography \ By Dr. G. Holst and Dr. L. Hamburger. 

 (Communicated by Prof. H. Kameklingh Onnrs). 



(Communicated in the meeting of September 29, 1917). 



1 . Introduction. 



The problem of the influence of small amounts of impurities upon 

 the emission of light by a gas, made us look for a method of 

 obtaining in a simple way a general survey of the alterations in 

 the emission of light. Through this means we arrived at a working 

 method very similar to the well known wedge-method ') used for 

 determining absorption spectra. Photographs obtained by the latter 

 method are very simple to read"), the height of the spectrum serving 

 as a measure for the absorption in the region of wave-lengths under 

 consideration. In analogy with this we arranged our spectrograph ') 

 in such a way that the height of the spectral line forms a measure 

 for the intensity of the incident light. For this purpose we introduced 

 a rotating sector with a radially increasing opening close in front 

 of the slit. The time of illumination increases therefore along the 

 spectral line from below upwards, and with it the blackness. The 

 most intense lines will become the longest on the plate ^). 



2. Arrangement of the apparatus. 



The sector must be so constructed that there is a simple connec- 

 tion between length of spectral line and the intensity of the incident 

 light. For this it is of the greatest importance to know, how the 

 blackness of the photographic plate depends upon the intensity I 

 and the duration of the illumination /. Under normal cii'cumstances, 

 according to Schwarzschild, the blackness is measured by the product 

 /^, where p is about 0.8*). With intermittent light the illumination 

 must be longer to produce the same blackness. The exponent becomes, 



1) H. Kayser. Handbuch der Spectroscopie, III. p. 58. 



2) Comp. for instance the atlases of absorption spectra by H. S. Uhler and 

 R. W. Wood, or by G. E. Kenneth Mees. 



3) HiLGER quarlz-spectrograph G. 



*) A similar arrangement for sensitometric purposes is used by E. Belin. 

 Brit. Journ. Phot (53) 630. 1906. 



'") H. Lux (Zeitsch. f. Bel. wesen 1917 p. 83) finds that in Wratten and 

 Wainwright plates, wliich we used also, p may vary from ± 0,88 to 6,3 while 

 according to L. Vegard Ann. d Phys. (".O) 111, 101'2 p = 0,89. 



71 



Proceedings Royal Acad. Amsterdam, Vol. XX. 



