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XXXVI. A Polarization Flicker Photometer and some Data 

 of Vheoretical Bearing obtained with it. By Herbert 

 E. Ives *. 



Synopsis. 



1. Introduction. 



2. Theory and Construction of a Polarization Flicker Photoinoter. 



3. Theory and Construction of a Mixture Photometer. 



4. Critical Speed Relations for various relative Brightnesses of Two 



Compared Fields. 



(a) Mean Brightness Constant. 



(b) Mean Brightness Varied. 



5. Measurement of the Brightness Difference Discrimination Fraction 



for Fluctuating Impressions. 



6. Critical Speed Relations with a Difference of Colour between the 



Two Fields under Comparison, 



7. Measurement of the Hue Difference Discrimination Fraction for 



Fluctuating Impressions. 



8. Relative Values of Brightness and Hue Discrimination Fractions 



under Steady and Fluctuating Conditions. 



9. Effect of Choice of Speed on Flicker Photometer Settings. 

 10. Summary. 



1. Introduction. 



IN previous papers by the present writer and Mr. E. ¥. 

 Kingsbury on "The Theory of the Flicker Photo- 

 meter "f, emphasis was laid on the desirability, from both 

 the practical and theoretical sides, o£ a flicker photometer in 

 which the transition from one field to the other should be 

 gradual and according to some simple mathematic relation. 

 Practically the advantage is in eliminating mechanical 

 flicker, due to dividing edges, which can introduce spurious 

 effects. Theoretically the advantage is that this kind of 

 transition, especially if it can be represented by the simplest 

 periodic function — the cosine curve — can be handled mathe- 

 matically J. 



The theory developed by us was based on a stimulus 

 represented by a cosine curve. The verification of the theory 

 by experiment, which was sought in work with flicker photo- 

 meters with abrupt transition, while very satisfactory, was 

 incomplete quantitatively in several details. We gave 

 reasons for believing that the outstanding discrepancies 

 were ascribable to the difference between the type of 



* Communicated by the Author. 



t " Theory of the Flicker Photometer," Ives & Kingsbury. Phil. 

 Mag. Nov. 1914, p. 708 ; April 1916, p. 290. 



% Other transitions can of course be represented mathematically ; the 

 difficulty is in solving the equations resulting when the diffusivity 

 conditions are introduced. 



