OF ARTS AND SCIENCES, 497 



This method seems to be quite sensitive. The change in resist- 

 ance is large when estimated by the number of ohms necessary to 

 restore a balance to the bridge. It was noticed that at a certain point 

 a comparatively small increase in heat radiations was accomjianied 

 by a large change in the amount of light emitted. This phenomenon 

 had been noticed early by Dr. J. W. Draper. One Leclanche cell with 

 five ohms in the circuit beside the resistance of the strip was sufficient 

 to raise the latter to a red heat, and precautions were then necessary 

 to prevent a change of resistance from the heating effect of the battery 

 employed with the Wheatstone's bridge. Being desirous of ascertaining 

 whether the resistance of the platinum wire changed after it had been 

 heated to a red heat and had been allowed to cool, I arranged the 

 resistance of the battery circuit outside the bridge, so that the wire 

 could be raised to a red heat, and then, having quickly weakened the 

 battery circuit, reraeasured the resistance of the strip. No difference 

 could be perceived in the resistance of the strip. Tliis illustrated the 

 fact discovered by Professor Langley, that thin strips of metal ar- 

 ranged as bolometer strips give up heat very quickly. 



The results of this experiment led me to think that a bolometer 

 strip of definite surface could be placed at a fixed distance from a 

 carbon loop of definite dimensions inside an exhausted glass vessel. 

 The amount of radiation which the bolometer strip receives could be 

 calculated ; and we might base our standard of light upon the point of 

 incandescence which would give a definite radiation at a fixed distance. 

 We could not distinguish by this method the energy produced by rays 

 of different refrangibility. It seems desirable, however, to substitute 

 for the uncertain estimation of colored lights by the eye an instrument 

 which will measure the energy produced by the radiating source at a 

 certain distance. Within certain limits I found that the bolometer 

 strip would indicate an increase or decrease of the amount of radiant 

 energy received while the difference in color of the incandescent lamp 

 made the observer at the photometer entirely uncertain of his measure- 

 ments. 



Owing to the difficulty of obtaining the proper apparatus for the 

 prosecution of the study of this method, I then studied the question of 

 the practicability of employing a thermopile to measure the amount of 

 radiation from an incandescent strip of platinum at a fixed distance. 

 Within a long photometer box was placed a thin brass vessel contain- 

 ing water. Steam was passed by means of a rubber hose into the 

 water of this vessel which was thus maintained at a constant tempera- 

 ture of about 94° C. The outside of the vessel was about 92° C. 

 VOL. XX. (n. s- XII.) 32 



