5<>4 C. Barus — Coronas of the Fog Layer 



Art. XLVIII. — The Independence of the Coronas of the Thick- 

 ness of the Fog Layer ; by G. Barus. 



1. Introductory. — As an adequate theory of coronas is yet to 

 be given, experiments with a definite bearing On the various 

 features of the phenomenon are desirable. In my earlier work* 

 I endeavored to elucidate the character of the interference phe- 

 nomenon superimposed on the diffraction phenomenon, whereby 

 the discs of coronas with white light eventually show a rhyth- 

 mic succession of colors. A theory was suggested correspond- 

 ing to that given by Verdet for the lamellar grating. Inferences 

 so deduced were quantitatively in accord with facts. I showed 

 that the reappearance of the same type of corona corresponds 

 to a succession of diameters of fog particles in the order of the 

 natural numbers n = 1, 2, 3, 4, etc. Finally, in a survey of 

 coronas obtained with monochromatic light (mercury), it ap- 

 peared that the disc and the first ring are alternately luminous 

 in a way corresponding to the interference phenomenon in 

 question. Finally, that in case of even the largest true coro- 

 nas fog particles of an order of size greater than 10~ ,cm were 

 in question, beyond which the corona degenerates into a 

 mere fog. 



In another paperf I touched upon the question of the inter- 

 ference with each other of coronas due separately to two suc- 

 cessive layers of fog particles normal to the line of sight, but 

 the quantitative relations did not seem to be as promising as 

 interferences inferred from the mere thickness of fog particles, 

 already alluded to in comparison with the lamellar grating. 



2. Effect of Thickness : Apparatus. — From the point of 

 view of the elementary theory the effect of the thickness of 

 the fog layer should be negligible ; but it does not by any 

 means follow that this is actually the case. In very many 

 experiments with coronas, the thickness of the fog layer is 

 not at the observer's disposal ; or cases of different thicknesses 

 have to be compared. Hence the following experiment was 

 devised, with the object of definitely testing the question. 



In the figure, FF" is a cross section of the fog chamber, 

 a long rectangular trough of wood, cloth-lined and provided 

 with two glass plates, g and g\ on the broad sides of the trough. 

 The pool of water is seen at w. Two mirrors, M and M', of 

 plate glass (their normals at n, n'), horizontally hinged at h and 

 hi and capable of being displaced parallel to their own plane by 



*See Carnegie Publications, No. 96, Part I, 1908; ibid., Part II, 1910. 

 This Journal, xxv, p. 224, 1908 (axial colors) : ibid., xxiv, pp. 309-12, 1907 

 (cycles of coronas); ibid., xxvii, pp. 73-81, 1909 (mercury light). 



f Proc. Am. Philos. Soc, April, 1911. 



