172 Reflective Power of Lamp- and Platinum-Black. 



The results for the reflective power of lampblack and 

 platinum-black are given in the following table : — 



Wave-length. 



Lampblack 

 (0-205 mm. thick). 



Platinum-black 

 (deposited 15 mins.). 



Platinum-black 

 (deposited 3 mins.). 





Apparent. 



True. 



Apparent. 



True. 



Apparent. 



True. 



0-8^ 



8-7u 



2-17 7o 



1-99 



2-04 



2-8 



3 



M37. 

 0-66 



0-67 



1-4 



1-6 



0-54 7 



0-98 



1-35 



1-21 



1-5 



0-17 7o 



0-59 



093 



079 



11 



1-34 7o 



5-80 



7-50 



7-2 



,s 



1-30% 



5-70 

 7-08 

 6-8 



74 ! 



25-5 ii..... 



51 n l 



51u 2 





1 Rock-salt Reststrahlen using metal shutter. 



2 Rock-salt Reststrahlen using rocksalt shutter. 



As a test of the method, the reflective power of polished 

 silver for fluorspar Reststrahlen was determined and the 

 value 94 per cent, was found. 



Kurlbaum's determinations of Stefan's constant were not 

 corrected for the reflective power of platinum-black. The 

 radiation from a black body at 100° C. has its maximum at 

 about X = 8 fi, where the reflective power has been found to 

 be 0*59 per cent. Hence Kurlbaum's value 



becomes 



7-061 xlO" 15 



7-10x10 



erg- 



cm. degrees 4 



era- 



cm. 2 degrees 4 ' 



Planck's value for the elementary quantity of electricity 

 is consequently increased in the same ratio^ and becomes, 

 taking Paschen's value for b *, 



4-621 xl0- 10 E.S. unit. 

 A reflective power of 1*0 per cent, would bring it to 

 4'65 X 10~ 10 , Rutherford and Geiger's f experimental value. 



I am greatly indebted to Prof. F. Paschen for suggesting 

 the investigation and for his helpful advice at all stages. 

 Physikalisches Institut, 

 Tubingen University. 



* 6=0-292, instead of Luuimer and Pringsheini's value, 0'294. 

 t Rutherford & Geig-er, Proc. Roy. Soc. A., lxxxi. p. 162 (1908). 



