RADIATION FROM RUBENS THERMOPILE. 79 



is allowed to radiate to what is generally the source, which is at a much 

 lower temperature. The first examination was made by Langley: 1 



He located the maximum galvanometer deflection in the region of 8 //, 

 but adds that the position of " the maximum depends upon a single obser- 

 vation of some delicacy, which is liable to subsequent correction." He 

 further adds that for this temperature difference of 18 the maximum 

 ought to be at 10 to 11 a. This would indicate that there was some con- 

 fusion of ideas as to what was really taking place. The energy emitted 

 by the vessel of ice and salt is very small in comparison to the amount it 

 absorbed from the bolometer. Hence the galvanometer deflections are a 

 measure of the radiation emitted by the bolometer to the vessel of snow 

 and ice, and the observed maximum is for a temperature of 2 instead 

 of 1 8 C. However, since a bolometer strip is always at a higher tem- 

 perature than the surrounding atmosphere, it is quite probable that, 

 although the observations were made during zero weather, the maximum 

 observed is really due to a higher temperature of the source (bolometer 

 strip) than 2 C. In fact, from the "displacement law," which was 

 then unknown, and from our present knowledge of radiation of different 

 surfaces, it is quite possible that the temperature of the bolometer was as 

 high as 20 to 30 C. 



Mendenhall and Saunders 2 state that they found the energy curve of a 

 complete radiator at 90 C. From their discussion of the energy curve 

 of a body at the temperature of boiling liquid air where the emission maxi- 

 mum should lie at 30 /1 (which, of course, is possible if one could reduce 

 the temperature of the radiometer below this point in order to make 

 measurements), it would appear that they, too, had not considered their 

 radiation curve to be due to the bolometer strip. Moreover, the use of a 

 bolometer to deduce a distribution of energy curve at low temperatures, 

 where the bolometer itself becomes the source, is not very satisfactory, due to 

 the fact that the temperature of the radiating strip can not be determined. 



Lummer and Pringsheim 3 obtained a small portion of the spectrum 

 energy curve of a screen at room temperature, which they used during 

 their radiation experiments. The first really lucid discussion of the sub- 

 ject is due to Stewart, 4 who found the spectrum energy curve of a Nichols 

 radiometer radiating to a receiver at liquid-air temperature. The maxi- 

 mum deflections observed were 4 mm. The temperature of the room 

 (and radiometer vane) was 24 C. or 297 abs. The observed maximum 



1 Langley: Amer. Jour. Sci., 31, p. 1, 1886. "The radiator was the bolometer itself at a 

 temperature of 2 C, and the source radiated to was a vessel of snow and salt at 20 C, 

 thus determining the distribution of energy in the spectrum of a surface below the freezing- 

 point of water." 



2 Mendenhall & Saunders: Astrophys. Jour., 13, p. 25, 1901. 



3 Lummer & Pringsheim: Verb. Phys. Ges., 2, p. 163, 1900. 



4 Stewart: Phys. Rev., 17, p. 476, 1903. (In this paper figs. 4 and 6 are interchanged.) 



