1039 



difficult owing to the great difference in colour. For this reason F 

 was measured without the filter. From the brightness obtained in 

 this way and the absorption of the filter the brightness of F with 

 the filter in front could be calculated. The absorption of the filter 

 was determined separately with a flicker-photometer for the same 

 spectral energy-distribution of the light-source. For this measurement 

 the intensity of the ground glass was too small, however ; according 

 to Ives ^) in order to obtain satisfactorj- results with a flicker-photo- 

 meter the illumination of the photometer-fields must be about 25 

 metre-candles. This higher intensity was obtained by cementing 

 together the two plates of ground glass with a drop of canada-'ualsam ^). 

 They became transparent thereby and the absorption in the bichro- 

 mate filter could now be determined at a suitable brightness of the 

 photometer-fields. Fig. 2 shows the ratio of the intensities with and 

 without filter at various current-strengths through the projection- 

 lamp. The measurements both with the flicker-photometer and the 

 Brodhun-photometer are given in the figure. Although the determi- 

 nations with the Brodhun-photometer will be seen to deviate mutually 

 more than those with the flicker-photometer, the two series do not 

 appear to give any systematic difference. This is not to be wondered 

 at considering that all the measurements were made at such inten- 

 sities of the photometer-fields that the Purkinje-effect could not have 

 any influence. 



1) Ives. Phil. Mag. (24) 1912. 



*) The glass-plates which we used were not completely free of colour. Other- 

 wise they might have been simply removed. 



72* 



