1907.] 



BARUS— NUCLEI IN DUST-FREE WET AIR. 



71 



method of a single point of light and a goniometer of special type 

 on opposed sides of the fog chamber was first used. Figs. 2 and 

 3 give examples of these results. 



Subsequently a new method was devised, in which two identical 

 sources of light equidistant from the eye, were moved symmetrically 

 toward and from each other, on a line parallel to the axis of the 

 fog chamber. Observation consisted in placing the fiducial annuli 

 of each of the two coronas in contact, by adjusting the lights at a 

 distance 5 apart. A lever for this purpose is in the hands of the 

 observer. The normal distance between 6^ and the fog chamber 

 being R and the angle of diffraction, S = 2R tan 0. The advan- 



la 



lb 



Fig- 



Fig. la. Diffractions from a single source S from fog particles a. b, c, 

 within the fog chamber, suggesting changes of the angle of diffraction to 

 an eye at e. r is the radius of the goniometer. 



Fig. i^. Diffractions from two sources S' and S" from fog particles 

 at a, b, and c, within the fog chamber. Coronas iin' and n'n" in contact. 



Fig. ic. The same drawn to scale. Fog chamber at F. Angles of dif- 

 fraction shaded. Coronas nn\ nn". 



