On a Polarization Flicker Photometer. 361 



flicker photometer theoretically postulated and that actually 

 used. 



In the present paper a flicker photometer is described in 

 which the transition from one light to another is exactly of 

 the character handled in the theory. It has been possible 

 with this photometer to verily accurately the predictions of 

 the theory. Measurements of the visual constants have 

 been made, and, as a consequence, the complete behaviour of 

 this type of equal exposure flicker photometer can now be 

 represented quantitatively by a set of equations derived from 

 the physical theory of conduction. 



2. Theory and Construction of a Polarization Flicker 

 Photometer. 



The polarization flicker photometer is a combination of a 

 double-image prism with a rotating Nicol prism. Two 

 images of each half of the photometric field are formed by 

 the double-image prism, and the dimensions of the apparatus 

 are so chosen that the horizontally polarized image of one 

 half is superposed on the vertically polarized image of the 

 other. The Nicol prism as it rotates alternately extinguishes 

 each image, the one fading gradually into the other, with 

 entire absence of any mechanical edge effect. 



Fig. 1. 



Polarization nicker photometer : N, rotating Nicol prism ; W, double- 

 image prism ; Fi and F 2 , two halves of photometer field. 



Alternative arrangement of polarization flicker photometer. 



Figs. 1 and 2 show two constructions. In fig. 1 the Nicol 

 prism is placed at the eye. When So used the nicol must be 

 o£ the type in which the faces are perpendicular to the beam 



Phil. Mag. S. 6. Vol. 33. No. 196. April 1917. 2 B 



