THE 



AMERICAN JOURNAL OF SCIENCE 



C P [THIRD SERIES.] 



<? — 



Art. XIX. — Some Determinations of the Energy of the 

 Light from Incandescent Lamps /* by Ernest Merritt, 

 M.E , Fellow in Physics at Cornell University. 



[Contributions from the Physical Laboratory of Cornell University, No. IV.] 



In the numerous experiments that have been made on the 

 efficiency, life, etc., of incandescent lamps, the light from the 

 lamp is always measured photometrically, and is expressed in 

 candle powers. For most purposes this method is satisfactory, 

 for it is the luminous effect of the light that is of practical 

 importance. It is of some scientific interest, however, to de- 

 termine what proportion of the energy supplied to a lamp is 

 given off as light, and what proportion is wasted, practically, 

 as dark heat. Photometric methods are evidently of no use 

 in determinations of this kind. The candle power is rather a 

 physiological than a physical unit ; it measures the effect of 

 light waves on the eye, and not the energy of these waves. It 

 was necessary, therefore, on beginning the investigation to be 

 described in this paper, to find some means of separating the 

 visible from the invisible rays, and of measuring each in units 

 of power. 



The most exact method would probably have been to form 

 a spectrum of the light to be tested with a rock salt prism, and 

 to measure the energy of the visible and invisible portions by 



*Read before the American Association for the Advancement of Science. 

 Cleveland, August, 1888. 

 Am. Jour. Sci.— Third Series, Vol. XXXVII, No. 219.— March, 1889. 

 11 



