1037 



Langmüir has made a series of measurements on tungsten-lamps 

 from which b}' means of the mean emissive power the briglitness 

 of the black body may be calculated. At the higher tem{)eratures 

 the results of these measurements are in good accordance with the 

 calculated values. At the lower temperatures the measured values 

 are considerably smaller. Langmuir ascribes this deviation to the 

 influence of the Purkinje-effect in the photometric determinations. 

 If the values found by Nernst and by Lummer and Pringsheim are 

 reduced to the temperature-scale used by Langmuir, the results of 

 their measurements are found to be vei-y nearly in a constant ratio 

 to those of Langmuir's; Langmuir finds an intensity which is 1.15 

 times higher. It is, therefore, probable that in the measurement of 

 the former investigators also the Purkinje-effect must have played 

 a part. Ives and Kingsbury^) have also pointed this out. It thus 

 seemed to us worth while determining the intensity of the black 

 body once more, arranging the method in such a manner that the 

 Purkinje-effect cannot arise. We made our measurements in the 

 neighbourhood of the melting-point of gold seeing that this tempe- 

 rature is sufficiently well ascertained to enable us from the intensity 

 as measured, the known energy-distribution of the black body and 

 the luminosity curve for the human eye to calculate the mechanical 

 equivalent of light. 



Method and instruments. 



The arrangement of the method is rendered in fig. 1. Through 

 the telescope A, the photometric prism B and the two totally reflect- 

 ing prisms C and D, a small disc of alundum blackened with oxy des 

 and placed in the furnace E and a plate of ground glass F are 

 seen immediately beside each other. F is provided with a diaphragm 

 the opening of which is measured with great accuracy. F is illum- 

 inated by a second plate of ground glass (rand a projection-lamp H, 

 whici) is placed behind it. The three prisms B, and C and D are 

 mounted on one plate /. They may be removed in a body and 

 replaced in exactly the same position. When 1 is removed the tem- 

 perature of the furnace may be measured with the Holborn and 

 KuRLBAUM-pyrometer J and the candle power of the ground-glass F 

 with the BRODHUN-sector photometer A^. 



In front of F an additional rotating sector L was placed. This 

 sector rotated during the comparison between furnace and ground- 

 glass but was stationary in the photometric measui-ernent of F in 



Iyes and Kingsbury, Phys. Rev. (8) 177, 1916 



72 

 Proceedings Royal Acad. Amsteidam. Vol. XX. 



