CHAPTER II. 



STANDARDIZATION AND EFFICIENCY OF THE FOG CHAMBER. 

 AXIAL COLORS AND INTERFERENCES. 



15. Introductory. The axial colors of the steam jet and of coronas 

 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 differ- 

 ence whether the source is a point simply or a disk (say) 4 inches in 

 diameter; it appears 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 wide drum i to 2 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. 



16. Causes of axial color. In my earlier work* I was inclined to 

 regard these colors as interferences superimposed on the coronas, regard- 

 ing the small field of refraction possible with small particles as in keeping 

 with the long columns needed for observation. The explanation at best 

 is purely tentative. Later in my work, when the size of particles was esti- 

 mated from data given by successive exhaustions,! it appeared that the 

 sizes 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 d and the ratio in question are somewhat reduced 

 but remain 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 inter- 

 ference colors, the results given in table 6 may be selected at random. 

 They show that the strong axial blues of the first order must belong to 

 particles even larger than o.oooi cm. in diameter, and that all particles 

 are more than six times larger than would be demanded for interferences. 



*Phil. Mag. (5), xxxv, p. 315, 1893; Bu U- U. S. Weather Bureau No. 12, 1895. 

 fPhil. Mag. (6), iv, p. 26, 1902; Smiths. Contrib. No. 1373, 1903; No. 1651, 1905. 



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