172 E. Merritt — Light from Incandescent Zamj)s. 



source rises. The correction due to this fact was determined 

 as follows : The radiations from the lamp, after passing 

 through the water and glass of the calorimeter, were allowed 

 to fall on the face of a delicate thermopile, and the deflection 

 of the galvanometer in circuit was observed. A small cell con- 

 taining an opaque solution of iodine in carbon disulphide was 

 then placed between the pile and the lamp, and the deflection 

 again observed. The iodine solution cut off the light entirely, 

 but allowed the dark heat to pass through. The ratio of the 

 second deflection to the first, therefore, gave the ratio of the 

 dark heat escaping from the calorimeter to the total radiant 

 energy that escaped. It was found that a little less than one- 

 third of the energy that had passed through the glass and 

 water of the colorimeter, was dark heat. This correction was 

 determined for each particular candle power at which the lamp 

 was run. 



The lamp used in the experiments was an Edison " 108 volt," 

 16 C. P. lamp with a cold resistance of 255 ohms. This lamp 

 was perfectly new when the tests began. Determinations of 

 the energy of the light were made at seven different candle 

 powers ; the results are given in the following table : 







Lamp A. 



Edison. 















L 



L 



E. M. F. 



W 



C. P. 



L 



W 



C. P. 



74-2 



34-6 



0-9 



0-18 



0-005 



0-59 



91-6 



56-2 



4-8 



0-68 



•012 



0-14 



97-3 



64-6 



7-3 



1-13 



•017 



0-15 



100-3 



693 



8-9 



1-62 



•023 



0-18 



107-6 



81-6 



14-6 



2-97 



•036 



0-20 



109-3 



84-4 



16-3 



4-57 



•054 



0-28 



124-1 



115-4 



38-2 



7-46 



•065 



0-19 



In this table W is the total energy, in Watts ; L the energy 

 of the light, also measured in Watts ; and 0. P. the candle- 

 power. 



Some of these results are shown graphically in fig. 3. The 

 curve A is found by taking as abscissae the different values 

 of W, and as ordinates the corresponding values of L. The 

 vertical scale is about ten times the horizontal. 



II. Determinations made with the Thermopile. 



I returned to this subject in 1888 and continued the investi- 

 gation by a somewhat different method. The calorimeter was 

 abandoned, and for absorbing the dark heat a cell one deci- 

 meter thick containing a strong solution of alum was used. 

 After passing through this cell the light was allowed to fall 

 on a thermopile, and the deflection was observed. Then the 



