.iHMN.icis Oh HI. 1. 1. srsriM iia iixic.u. I'.iri.Rs m.\ 



ustnl to Kind a valiH- fur tin- U-ast nu'rhaiiic.il r(nii\'.iU-nl of li^jlil iisinj; 

 thi- latfst willies for tlu- Itl.ick Ixxly ronstaiils and llu- inciting point 

 of plaliiniM). Till- s|HTtral luminous I'lTicicncy curxi.' ol>laiii(.-(l by 

 T\iulall ami (iil)son is cuiployrd. It is found that omt tlii- t-nliri' 

 range of probable values of the black bod\- constants, the values for 

 the least mechanical e(|uivalent of IIkIu may be plotted as a straigln 



line in liTtns of ' sn that llic prix'nl conipiii.ii ions may be I'x- 



pressed in a simple e(iiiation in which any desired \aliies of the black 

 Ixnly constants may be inserted. I'sinj; the latest \-alues the least 

 mechanical equiN'alent of light is found to be .OOHil watts per lumen. 

 This is practicalK' iilentical with the value obtained by using the 

 author's earlier experimental determination using the monochromatic 

 green mercury light, when combined with the (iibson and Tyndall 

 liimuious efticiencN' cur\e. 



Pholoelfdric Properties of Thin Films of Alkali Melals.' HisREiicr i 

 I-".. Ivi-:s. The thin hlms of alkali metals which deposit spontaneously 

 on clean metal surfaces in highh' exhausted inclosures are studied. 

 The alkali metals, sodium, potassiimi, rubidiimi. and caesium, in the 

 thin film form all exhibit, to a striking degree, the selectise plioio- 

 electric effect first discovered in sodium-potassium alloy. Kxperi- 

 ments on \arying the thickness of the deposited film show that the 

 selective effect only occurs at a certain stage of the tilni's (li\(l()[)- 

 ment; for \-ery thin films the selecti\'e effect is absent, .iiid il dis- 

 appears again for thick layers of the pure alkali metal. The wa\e- 

 Icngth maxima of emission previously ascribed to the selecti\e effect 

 in the pure alkali metals on the basis o{ observations with rough or 

 colloidal surfaces are absent in these thin films. 



The Normal and Selective Photoelectric Effects in the Alkali MrUtIs 

 and Their Alloys.*" Hkrbkrt E. Ivks and A. L. Johnsri d. The 

 photoelectric currents from specular surfaces of molten sodiiuu. pol- 

 assiimi. rubidium, and caesium, and their alloys are studied at 

 \arious angles of incidence for the two principal planes of polariza- 

 tion. The selective photoelectric effect is clearly exhibited only in 

 the case of the lifjuid alloy of sodiimi and potassium. Wave-length 

 distribution curves show maxima of emission, which are usually, but 

 not always, most pronounced for light polarized with the electric 

 vector parallel to the plane of incidence. The wave-length maxima 

 previously assigned to the several elements are not confirmed; the 



' .\strophysiral Journal, Vol. LX, No. 4, November, 1924. 

 "" .Xstrophysical Journal, Vol. LX, No. 4, November, 1924. 



