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XXI. The Reflective Power of Lamp- and Platinum- Black. 

 By T. Hoyds, M.Sc, 1851 Exhibition Science Scholar *. 



ANGSTROM found in 1898 f that the reflective power of 

 a thickly sooted platinum-black surface was from 

 0*82 to 1*25 per cent, for different regions of the spectrum. 

 On the other hand, Fery J concluded from his experiments 

 that as much as 18 per cent, of the radiation from a black 

 body at 100° C. was reflected from a platinum-black surface, 

 and approximately the same amount from a lampblack 

 surface. Since a knowledge of the reflective power of the 

 surfaces usually employed as receivers of heat radiation is 

 important for the determination of the absolute radiation 

 constants, a method suggested by Prof. Paschen § has been 

 employed to determine the reflective power of lamp- and 

 platinum-black for definite wave-lengths in the infra-red. 



The method consists in measuring the galvanometer 

 deflexions, first when rays fall directly on a thermopile, and 

 then when they fall on the lamp- or platinum-black, the 

 rays diffusely reflected from it being focussed on a thermo- 

 pile by means of a polished hemisphere of german-silver. The 

 black surface A (fig. 1, p. 168) was attached alongside the ther- 

 mopile slit B which was situated immediately in front of the 

 exposed junctions. An external concave mirror cast, through 

 a narrow opening cut in the hemisphere, an image of an illu- 

 minated slit on to the thermopile when placed at C a little to 

 the left of the centre D of the hemisphere. If the thermopile 

 together with the black surface was now displaced relative 

 to the hemisphere until the thermopile came to B at an equal 

 distance on the opposite side of the centre, the slit image 

 would then fall on the black surface and the reflected light 

 would be focussed on the thermopile by the hemispherical 

 mirror. The galvanometer deflexions in these two positions 

 of the thermopile would measure the intensities of the 

 incident and reflected radiation respectively. In order to 

 interchange different black surfaces or to interpose screens 

 in front of the thermopile only, the thermopile with the 

 surface attached could be taken out and afterwards replaced 



* Communicated by the Author. A preliminary communication 

 appeared in the Physikalische Zeitschrift, xi. p. 316 (1010). 



f Angstrom, Ofversigt of K. Vetensk.-Akad. Forhandl. Stockholm, v. 

 p. 283 (1898). 



X Fery, C. R. 148. p. 777 (1909). 



§ Modification of method bv which Paschen made his bolometer 

 blacker. See Ber. Berl. Akad. d. Wiss. April 27th, 1899. 



