362 Mr. H. E. Ives on a 



of light, otherwise the whole field will oscillate with the 

 rotation of the prism. In fig. 2 the nicol is placed away 

 from the eye and beyond the diaphragm which limits the 

 field. In this arrangement the nicol may be of the oblique- 

 faced type. The apparatus actually used was arranged as 

 in fig. 2, because of the kind of nicol prism available. The 

 distances were such that the photometric field had an angular 

 aperture of 1J degrees. The artificial pupillary aperture was 

 3 mm. in diameter. 



The theory of the polarization flicker photometer is simple. 

 Let 2I be the brightness of the image polarized in one 

 plane, then the brightness of the image when the nicol is 

 rotated through the angle <£ from that plane is 



2I cos 2 <£. 



Now 2l cos 2 <£ = I (1+ cos 20) . . . . (1) 



If we put 2(p = a)t, the right-hand member of (1) becomes 

 the right-hand member of equation (1) in the first theoretical 

 paper, that is, we have exactly the type of stimulus there 

 postulated. 



3. Theory and Construction of a Mixture Photometer. 



In the discussion of colour flicker it was shown that the 

 action of the flicker photometer is to form a mixture of the 

 two colours under measurement. The speed at which it is 

 necessary to run the flicker photometer in order that there 

 shall be no colour flicker is that at which the hues of the 

 extreme mixtures differ less than the least perceptible hue 

 difference. The relationship of this hue difference to the 

 least perceptible brightness difference and the relationship 

 of both of these to the corresponding quantities for steadily 

 viewed juxtaposed fields, are crucial points in flicker photo- 

 meter theory. 



In order to be prepared for hue difference perception 

 measurements the photometer " head," if it may be so called, 

 was made in such a way that the light from the two sources 

 under comparison could be mixed in any desired proportions. 

 The apparatus is shown diagrammatically in fig. 3. It 

 consists of two translucent (flashed opal) glasses, arranged, 

 one over the other, to turn by means of gears in opposite 

 directions, about a common axis parallel to their faces. 

 This device is set up at the intersection of two photometer 

 tracks, standing at right angles to each other, and is viewed 

 from the rear. If used for equality of brightness obser- 

 vations a biprism is employed to juxtapose images of the 



