112 Optical Properties of Films of Sodium and Potassium. 



excess in the spectra (see Wood's c Physical Optics/ 

 Laminary grating). 



If, now, the grating space be made sufficiently small, the- 

 lateral spectra disappear and Xmust appear again in the central, 

 image, notwithstanding the half-wave retardation. Lord 

 Rayleigh expresses the opinion that in this case the narrow r 

 grooves may, perhaps, act as resonators. So far as I know 

 this experiment has never been tried, and it is my plan to- 

 attempt it with the 110 fi waves obtained by focal isolation 

 with quartz lenses. 



It appears to me possible that the cavities may be found 

 to act as absorbing resonators, X still remaining absent in 

 the reflected light, in which case we should have something 

 quite analogous to the behaviour of the sodium films, though 

 I am very doubtful about this, however. 



Whether the phenomena exhibited by sodium depend upon 

 the form of the cavities alone or whether the optical pro- 

 perties of the metal must be taken into consideration remains 

 to be seen. A qnantitive investigation of the transmission 

 and reflexion of films of the alkali metals is now in progress 

 in our laboratory under the direction of Dr. Pfund, which 

 will furnish some of the required data. 



It seems to me very important to establish a dynamical 

 theory based on a theoretical investigation of the behaviour of 

 surfaces covered by cavities small in comparison with X, with 

 respect to sether waves. Up to the present time no such 

 investigation has been made, so far as I am aware. 



My feeling at the present time is that the cavities trap the 

 7-adiation in some way and cause almost complete absorption 

 for certain values of X. An inspection of the curves shows 

 that a change of X from 70 to 50 may cause the reflecting 

 power to drop from 75 per cent, to 4 per cent. 



This enormous change, combined with the fact that the 

 reflexion remains high in the remainder of the spectrum 

 (i. e. in the ultra-violet), makes it appear certain that 

 we are dealing with a phenomenon which cannot be 

 explained by any of the simple laws of interference and 

 diffraction. 



