Theory of the Flicker Photometer 



313 



as indicated on the respective curves. All values are calcu- 

 lated from the experimental value of 35 cycles per second 

 for the 180-degree case, using the value — 3 for log S. 



These curves show clearly one point which appears at once 

 on experiment, namely, the lower sensibility of the un sym- 

 metrical instrument. There is, too. a somewhat greater lack 

 of symmetry about the equality point than with the equal 

 exposure arrangement. 



Fig. 11 exhibits as well in convenient form the phenomena 



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Calculated sensibility characteristics of Flicker Photometer with 

 unequal exposures of the compared lights. One fixed, one variable 

 illumination. Variable illumination exposed 30°, 180°, 330°. 



visible in a flicker photometer in which the alternation of 

 the two lights is performed by an oscillating lens or mirror 

 whose excursions are too short. While the centre of the 

 field of such an instrument is equally exposed to the two 

 lights, one Fide is exposed longer to one light than the other, 



