90 O. Bar us — Standardization of the Fog Chamber. 



important constant. For this purpose I am at work on a 

 redetermination of the nucleation values of the coronas, using 

 as a source of light the virtually monochromatic mercury 

 lamp. This is sufficiently intense and the coronas admit of 

 the more definite optical interpretation. 



Elsewhere* I pointed out that for large coronas the greater 

 part of the fog particles evaporate ; thus even at n — 200,000 

 particles per cubic cm., about one-half evaporate and one-half 

 subside. Hence the corona method is here alone available for 

 counting particles. I also showed that in the case of coronas 

 the interference phenomenon superposed on the diffraction 

 phenomenon may be treated in a way similar to the lamellar 

 grating, consisting of alternate strips of thin and thicker trans- 

 parent glass ; that the given types of coronas must follow 

 each other in the ratio of 5, 4, 3, 2, 1, for their particle 

 diameters and an increasing size of coronas ; that the ratio of 

 fog particle diameter and interference plate thickness, d/D, for 

 the same color minimum in the interferences is d/D = n/(n-l), 

 where n is the refractive index, or about 7 to 8 in both cases. 

 It must therefore be possible to compute the nucleation cor- 

 responding to a given corona at a given exhaustion and tem- 

 perature, purely from optical considerations of diffraction and 

 interference, as indicated. I hope to report the results in the 

 near future. 



Brown University, Providence, E. I. 



*This Journal, xxv, p. 409, 1908. flbid., p. 224, 1908. 



