214 REPORTS OF INVESTIGATIONS AND PROJECTS. 



(2) In a succeeding investigation Dr. Barus made further studies of the 

 optics of coronas. It was shown that the interference phenomenon superim- 

 posed upon the diffraction phenomenon in the case of coronas may be treated 

 in a way similar to the lamellar grating, consisting of a uniform succession 

 of alternate strips of thin and thicker transparent glass. Given types of 

 coronas are reproduced in successively increasing size, when the respective 

 fog-particle diameters are in the ratio of . . . . 5, 4, 3, 2, i, o. The ratio of 

 fog-particle diameter d and interference-plate thickness D for the same color 

 minimum in the interferences and a film of water is d / D = n (n — i), 

 where n is the index of refraction of water. The experimental values of 

 d / D agree well with this. It must therefore be possible to compute the 

 nucleation corresponding to a given corona, purely from optical considera- 

 tions of diffraction and interference, as indicated. To further verify the 

 theory suggested, special study was made of the axial or interference colors 

 of coronas, by the aid of large drum-shaped chambers 2 meters long. 



The coronas obtained with electric light are almost too complicated for 

 practice, for which reason a part of the mantle of a Welsbach burner has 

 usually been used as a source of light. Much better results are obtained, 

 however, by the use of the virtually monochromatic mercury lamp as a 

 source. This is sufficiently intense and admits a more definite optical inter- 

 pretation. Experiments have therefore been in progress to standardize these 

 simplified coronas by the method of successive exhaustions and phosphorus 

 nuclei. The work is now practically finished and all anticipations have been 

 realized. 



(3) In an endeavor to standardize the coronas in terms of the nucleation 

 involved, by the aid of separate small batches of sealed radium, used singly or 

 in groups, very little progress was made, because the coronal diameter varies 

 as the sixth root of the intensity of ionization. The experiments, however, 

 lead to certain remarkable results on the distribution of ionization, with ref- 

 erence to the position within the fog-chamber of the sealed aluminum tube 

 (beta and gamma rays being in question, largely the latter) containing radium. 

 If the parts of the fog-chamber consist of different materials or not, the 

 maximum ionization due to primary and secondary radiation rarely coincides 

 with the position of the radium. In a horizontal cylindrical fog-chamber, 

 closed at one end and open for exhaustion at the other, the maximum ioniza- 

 tion is found to move from the closed end to the exhaustion end, as the ra- 

 dium moves from the closed end to the middle of the chamber. As the radium 

 moves further the maximum remains near the exhaustion end, but the ion- 

 ization diminishes in marked degree throughout the whole chamber. The 

 ratios of ionization are frequently greater than 2 to i. To obtain maxima of 

 ionization near the middle of the chamber the sealed radium tube must be 

 near the closed end. In other adjustments even minimum ionization was 

 producible in the middle, as compared with the ends. It appears from the 



