190 A. Pedler — On the use of the U%cUometer as a JPhotometer. [August, 



proportional to the square of the distance from the source of light. But 

 it will also be noticed that there is one marked exception to the rule, and 

 this is at a distance of 10 inches from the gas flame where the rapidity of 

 revolution is great. Here the actual number of quarter-revolutions amount- 

 ed to 59'36 per minute, whilst theoretically they should have been 90. 

 Evidently when the radiometer is rotating rapidly, there must be an im- 

 mense increase in the friction so as to reduce the rotation by one-third of 

 the whole amount. There are also indications in the above table that when 

 the radiometer is rotating very slowly, there is a considerable disturbance 

 from the theoretical rate of revolution, probably showing that friction has 

 much influence both when the rotation is slow and rapid. 



As the result of these observations was not very satisfactory in so far 

 as the applicability of the radiometer to photometry is concerned, I aban- 

 doned my former idea, that both the light and heat rays should be allowed 

 to act upon the radiometer ; and a second series of observations was com- 

 menced, in which the dark heat rays from the gas-jet were filtered off by 

 passing the light through a glass cell one inch in thickness with parallel sides, 

 which was filled with a saturated solution of alum. The method of obser- 

 vation was similar to that before described, and under these circumstances 

 it was found that the rapidity of the revolution of the vane fell very consi- 

 derably ; this will be seen from a comparison of the following table with 

 the preceding one. 



DarTc Seat rays cut offhy Alum solution. 





Illumina- 



Eadiometer in quarter-revolutions pei 



minute. 





ting power 















of gas-jet. 



10 in. 



15 in. 



20 in. 



25 in. 



30 in. 





14-30 



42-5 



24 



14-5 



8-5 



5-0 





13-30 



40 



22-5 



12-5 



8-0 



4-5 





15-52 



48 



26 



16 



11-5 



6 





13-10 



39-5 



23-5 



14-5 



9-5 



6 





13-76 



44-5 



31 



17 



9-5 



5-75 





12-79 



42 



24-5 



14 



8-5 



5-0 



Average, 



13-8 



42-75 



25-25 



14-42 



8-92 



5-37 



Calculated to 10 per min. 



at 30 inches distance, . . 



Theoretical, • 



} •• 



79-6 



47'0 



26-8 



16-6 



10 











90 



40 



22-5 



14-4 



10 





11-5 



25 



13-5 



8-5 



5 







9-1 



20-5 



11-0 



6 









9-56 



18-5 



13-0 



7-5 







The velocity of rotation has, it will be seen, fallen to about one-fourth 

 of what it was previously, and this is due almost entirely to the absorption 



