60 Sir John Conroy. On the Amount of Luminous and 



clearly at about 120 on the scale, and its value, as deduced in the 

 manner above described, is 118 5. From the face of the thermopile 

 to the zero of the scale was 40 cm. ; this quantity had, of course, to 

 be added to the scale readings of the position of the lamp. The 

 distance of the axis of the lamp from the face of the thermopile, when 

 the cell was interposed, was 31*8 cm. ; the glass and water of the cell 

 being more refractive than air, interposing the cell virtually reduced 

 the distance between the pile and the lamp. The rays which 

 traversed the cell differing in refrangibility, and being incident upon 

 the glass at different angles, the simplest form of the formula 



x = e (\ — -\ (in which n was taken as 1*33, and the glass and 

 \ nj 



water treated as if they had the same index) was used for calculating 

 the optical shortening of the distance. 



The values of x for the four cells were 0'4 mm., 1*4 mm., 2v mm., 

 and 4 mm. ; these amounts subtracted from 31'8 cm., gave the virtual 

 distances in the four cases. 



Calling the two distances of the lamp d± and d 2 , the intensity of 

 the lamp I, and the coefficient of transmission K, 



then 



s , whence K = ^J. 



Table III gives the results of measurements made in this way with 

 the different cells. Column 2 gives the values of e^, that is, the 

 scale reading of the position of the lamp without the cell, plus the 

 distance between the zero and the pile ; column 3 the corrected dis- 

 tances with the cell, i.e., the measured distance less the correction 

 for the refraction of the cell, and column 4 the coefficients of trans- 

 mission. 



Table III. 



d t . d 2 . K. 



Cell 1 154 31-4 04157 



148 31-4 04501 



158 5 31-4 0-03925 



0-04194 



Cell II 204 30-4 02221 



200-0 30-4 0-02310 



208-5 30-4 0-02126 



02219 



Cell III 215 29-1 01832 



225-0 29 1 0-01673 



224 29-1 01688 



0- 01731 



