606 



Prof. F. Allen on a New Method of 



frequency of flicker of both white and coloured lights was 

 taken to mean equality of brightness. The brighter the 

 spectral light the greater was the rotation of the polarizer 

 required, since a smaller portion of light was sufficient in that 

 case to equal the luminosity of the standard of comparison. 

 The luminosity of each part of the spectrum is inversely pro- 

 por cional to the intensity of the portion of light passing- 

 through the nicols, which is proportional to the square of 

 the cosine of the angle between the principal sections of the 

 prisms. 



Observations were made upon nineteen portions of the 

 spectrum. These are given in Table I. The results are 

 shown graphically in the luminosity curve in fig. 2. 









Table I. 







^ngle 













between 





i 



Eeduced to 





A. 



planes of 



IHCOls = rt. 



cos- a. 



cos' 2 a 



max. = 100. 



Kemarks. 



•414/1 



0° 



1 



1 



0-16 



Standard of comparison. 



•442 



9° 



•975 



1-02 



0-16 





•460 



35° 



•671 



1-49 



24 





•480 



63° 



•206 



4-85 



0-78 





•491 



70° 



•117 



8-55 



1-42 





•500 



76° 45' 



•052 



19 05 



3-10 





•515 



83° 



•0148 



67-34 



11-1 





•530 



84° 54' 



•0079 



12653 



20-6 





•564 



87° 



•0033 



365-4 



59-4 





•593 



87° 40' 



•0016 



603-5 



98-0 





(•598) 

 •601 







(615) 

 603-5 



(100) 

 98 



JSot observed. 



87°" 40' 



■*6bi6 





•621 



87° 15' 



•0023 



434-8 



70-6 





•648 



86° 



■0048 



205-5 



33-4 





•663 



84° 



•0109 



91-57 



15-2 





•677 



79° 



•0364 



27-47 



4-47 





•695 



73° 24' 



•0816 



12-25 



1-99 





•712 



55° 24' 



•322 



3-1 



0-50 





•733 



37° 30' 



•629 



1-59 



025 





•748 



0° 



1 



1 



- 0-16 



/ Equal to standard of 

 \ comparison. 













In plotting the curve the maximum ordinate is given 

 an arbitrary value of 100, and the oihers are reduced 

 proportionately. At least three independent observations 

 were made on each colour, and these always agreed with 

 each other very closely. In thirteen of the nineteen cases 

 the settings of the polarizer differed from the mean value by 

 less than one per cent. ; in the remaining six the differences 

 were greater. 



The ordinates of the curve do not represent absolute lumi- 

 nosities of the spectrum, but only values relative to the 



