Wedge-and-Diaphragm Photometer. 25 



fraction is, of course, reflected and lost; the remaining fraction 

 (s) enters the wedge. Let I be the illuminating-power of the 

 light (say a standard candle) at a unit distance from it, in 

 the direction of the photometer, n the thickness of the wedge 

 at the point interposed, m its coefficient of transmission, and 

 d the distance of the light from the opal pane in the slit; 

 then the illuminating-power of the light which reaches the 

 opal pane will be 



slm n 



IF' 



This light falls upon the surface of the opal glass; a fraction 

 enters it, part of which is absorbed, the remainder passing to 

 the mirror, a further part being absorbed in the reflection to 

 the eye of the observer. After the further diminution up to 

 this point the fraction of the light which leaves the opal pane 

 and which actually reaches the eye is s 19 the apparent illumi- 

 nation of the mirror with the smallest diaphragm being 



_ ssj>m n 



Similarly, if we now remove the candle and substitute some 

 other light whose illuminating-power at unit distance is L 

 and its actual distance D, the wedge will have to be read- 

 justed to n x i and if beyond the range of the wedge, the field 

 will have to be adjusted by the diaphragm being increased 

 (say to the illumination of the field \ 2 ) ; then 



_ ssjLm"! 



W 



x 2 — 



the relation of the illuminating-powers of the two lights will 

 be 



L /D\ 2 X* 

 I ~\d) Xj 



m n ~ n K 



The value of r^ is ascertained experimentally by obtaining a 



balance with some constant light at different distances, as well 

 as by measuring the openings of the diaphragms. The values 

 d and m" for the standard candle are constants. 



Let -w = a, then T 



dr L - X 2 a 



I \ x m n i 



D 2 . 



The wedge is provided with a scale divided into millimetres, 

 by which the position is observed. A table is constructed 



