STANDARDIZATION AND EFFICIENCY OF THE FOG CHAMBER. 17 



than the axial colors and are seen distinctly through very small thick- 

 nesses of fog; disk colors are apparently abruptly complementary to the 

 axial colors and 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 interferences 

 by transmission, there were complementary interference 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. 



19. Experiments with long tubes. In a long chamber and intense 

 illumination the axial colors may be extended over a considerable area 

 and intensified by strong illumination. It did not seem improbable that 

 they might then be serviceable for spectroscopic investigation, in which 

 case the mean wave-length of the interference bands would serve for 

 their identification. Thus they might afford a means of further investi- 

 gating the fog phenomenon at a degree of fineness beyond which the 

 coronas cease to be available. Unexpected difficulties were, however, 

 encountered, as will presently appear, and the endeavor to remove them 

 has not been successful. 



Roocm- 



FIG. 2. Section of brass tube for observing 

 axial color. 



Experiments of this kind were begun by using long brass tubes (fig. 2) 

 with plate glass ends, carefully put together. Sometimes wet cloth linings 

 were introduced; but they had no other effect than to dispel the hori- 

 zontal columnar vortices seen on each side of the axis after exhaustion. 

 With a naked tube the fog observed rises on the outside and falls in the 

 center of the field, so that the axes of the two vortical columns are eccen- 

 trically placed parallel to the axis of the tube. Phosphorus nuclei, ions, 

 and vapor nuclei were tried, and incidentally some fog and rain limits 

 determined. 



The following summary of results with vapor nuclei or with ions (dp 

 being the drop in pressure) , shows that in no case was there any color 

 obtainable. In these narrow tubes the only manifestation is a more or 

 less densely black fog. 



