226 G. Barus — Axial Colors of the Steam Jet and Coron 



as 



there is certainly no continuous transition ; finally there is no 

 incident light of the requisite obliquity. 



The appearance is therefore as if, corresponding to the inter- 

 ferences by transmission, there were complementary interfer- 

 ences by reflection toward the source of light. This phenomenon 

 could then be reversed in direction at any fog particle in its 

 path, and thus turned again toward the observer. But apart 

 from the complementary nature of disk and axial color, no 

 other evidence bears on this explanation. Moreover, any such 

 theory must account for the intensity of disk colors in general, 

 and in particular for the vividness of the greens. 



5. In a long chamber and intense illumination, the axial 

 colors may be extended over a considerable area and intensified 

 by strong illumination. It is not improbable that they will 

 then be serviceable for spectroscopic investigation, in which 

 case the mean. wave length of the interference bands ma} 7 

 serve for their identification. They would then offer a means 

 of further investigating the fog phenomenon at a degree of 

 fineness beyond which the coronas cease to be available. 

 Experiments of this kind are in progress. 



Brown University, Providence, E. I. 



