168 E. Merritt — Light from Incandescent Lamps. 



means of a bolometer. The elaborate character of the experi- 

 mental preparation necessary to the successful carrying out of 

 the bolometric method was, however, such as to necessitate its 

 rejection, and recourse was had to a modification of the method 

 first used by Melloni." He separated the light from the dark 

 heat by passing the radiations to be measured through a thin 

 layer of water, or better still through a solution of alum in 

 water. While the dark heat is almost entirely absorbed, the 

 light nearly all passes through. The energy of the light can 

 then be measured by a thermopile, and that of the dark heat 

 by the rise in temperature of the water. 



I. Color imetric Method. 



The first experiments which I shall describe were made by 

 Mr. S. Ryder and myself in the spring of 1886. The lamp to 

 be tested was placed in a large glass calorimeter, and main- 

 tained at the desired degree of incandescence. The greater 

 part of the light escaped through the glass, but the dark heat 

 was absorbed by the water and raised its temperature. The 

 energy of the dark heat could thus be measured. Corrections 

 were made for radiation, and for the absorption of light by the 

 calorimeter. The total energy supplied to the lamp being 

 determined by electrical measurements, the difference between 

 this total energy and the energy of the dark heat gave the 

 energy of the visible rays. 



The calorimeter was cylindrical in form, being 22 0m in diam- 

 eter and 38 cm high. It held about ten liters of water, and was 

 supported by two narrow bars of iron, which offered a quite 

 inappreciable obstruction to radiation. 



During the progress of the experiment, distilled water was 

 allowed to flow through the calorimeter in a steady stream, 

 entering at the bottom through the glass tube E [fig. L], and 



passing out at the top 

 through the tube F. The 

 temperature of the water 

 when entering and leaving- 

 was determined by means 

 of the thermometers C 

 and W, placed in the en- 

 trance and exit tubes of 

 the calorimeter. 



If the current in the 

 lamp, and the rate of flow 

 of the water, remain con- 

 stant, then the difference 

 in temperature between the water entering and that leaving 



* Melloni: " La Thermockrose," Naples, 1850. 



