224 C. Barus — Axial Colors of the Steam Jet ami Corona*. 



Art. XXIV. — The Axial Colors of the Steam Jet and of 

 Coronas / by C. Baeus. 



1. These colors overlie the source of light when looked at 

 through a long column of wet air in which uniform cloud 

 particles are suspended. It makes no difference whether the 

 source is a point simply, or a disk, say, four inches in diameter ; 

 it appeal's uniformly colored, as if seen through colored glass, 

 so long as the cloud lasts. The order of colors beginning with 

 particles of extreme smallness is the same as that, of Newton's 

 interferences, seen by transmitted light. In case of the steam jet, 

 however, on passing the transition from crimson to violet in 

 the first order, the field becomes opaque while the steady flow 

 of the jet usually breaks down and becomes turbulent. In the 

 case of coronas I have thus far failed to reach this transition, 

 the medium showing mere fogs of uncertain character. 



To produce the actual colors vividly, and especially the tints 

 of the second and third orders for relatively large particles, 

 the columns of fog must be long and very uniform. The steam 

 jet soon fails in this respect, but a drum one to two meters 

 long used as a fog chamber shows saturated colors surrounded 

 by coronas-. In the case of hydrocarbon vapors, the columns 

 may be shorter, because the particles throughout are larger 

 for like numbers per cubic centimeter, than is the case with 

 water vapor. 



2. In my earlier work* I was inclined to regard these colors 

 as interferences superimposed on the coronas, regarding the 

 small field of refraction possible witlrsmall particles, as in keep- 

 ing with the long columns needed for observation. The explan- 

 ation at best is purely tentative. Later in my work when the 

 size of particles was estimated from data given by successive 

 exhaustions, f it appeared that the size of the fog particles were 

 of an order about ten times larger than would be needed to 

 produce interferences of the same kind. The interference 

 hypothesis was therefore abandoned. In my more recent 

 results, the diameter of fog particles, a 7 , and the ratio in question 

 is somewhat reduced, but remains of the same order. Thus if 

 n be the number of fog particles per cubic centimeter, D the 

 thickness of an air plate giving like interference colors, the 

 following results may be selected at random. 



*Pkil. Mag. (5), xxxv. p. 315, 1893, Bull. U. S. Weather Bureau, No. 12, 1895. 

 f Phil. Mag. (6), iv, p. 26, 1902, Smith's contrib. No. 1373, 1903 ; No. 1651, 

 1905. 



