Cosine Flicker Photometer. 



645 



should be mounted on a photometrical bench in the usual way 

 for accurate work, the author has used the apparatus shown 

 in the Plate for man)" purposes, such as the comparison of 

 the results obtained by the methods of flicker and equality of 

 brightness, without employing a photometrical bench at all. 

 Also it is evident that the convenience of an instrument of 

 this type is independent of the distance between the sources 

 of light, provided the illumination of the photometrical 

 surface is not too low. 



The general principle of this instrument is shown in fig. 2. 



<$ 



Fiff. 2. 







<§> 



4- 



E 



.M 



Q 



Side View 



fRor 



View 



The two sides of the rectangular Ritchie wedge, W, are 

 illuminated by the two sources A, B. Above the wedge a 

 45° mirror, M, is placed so that the observer's eye at E sees 

 an image of the illuminated surfaces reflected in this mirror. 

 The wedge is arranged to rotate about the line of intersection 

 of the photometrical surfaces as a horizontal axis. To the 

 observer, therefore, this line appears stationary as the wedge 

 is rotated. 



Suppose now that the two sources A, B, are equidistant 

 from the wedge. 



Then, if the intensities of the two sources are the same, 

 photometrical balance is obtained when the wedge is placed 

 symmetrically about a vertical axis, as shown. If, however, 

 A, say, is the brighter of the two sources, the wedge must be 

 rotated in a clockwise direction, so that the rays from A strike 

 the surface presented towards A more obliquely than before, 

 while, conversely, the rays from B strike the surface presented 

 less obliquely. 



Hence let « represent the inclination of either surface to 

 the vertical, when the wedge is in its symmetrical position. 



Let Ij, I 2 represent the intensity of A and B respectively. 



The illumination of the surface facing A is then l 5 - . 



