72 BARUS— NUCLEI IN DUST-FREE WET AIR. [April 20, 



tage of this method of contact consists in this, that the observed 

 diffracton takes place in the equatorial plane of the fog chamber; 

 there is less obliquity of rays, and coronas of any size are observable 

 (an essential condition since the angle for the large coronas ap- 

 proaches 60° ) , and both eyes may be used, placed all but in contact 

 with the fog chamber. This diminishes the eyestrain and insures 

 sharper vision. Lantern slides were shown giving all the details 

 of this apparatus. The accompanying diagrams la and lb indicate 

 the differences of the old and new method and the latter is addition- 

 ally elucidated in figure ic. 



3. The Green Coronas. — If the coronas be divided into two 

 general types, those which have red discs or red primary annuli, 

 and those in which the discs are green, the latter are convenient for 

 comparison. In successive identical exhaustions they occur at 

 regular intervals and among the larger coronas three successive 

 series are particularly vivid, corresponding to fog particles whose 

 diameters are d^ = .00052, d^ = .00040, dc, = .00023 cm. These 

 numbers may be regarded as in the ratio of 4, 3, and 2, and they 

 suggest a first series with c^i = . 00013 as the highest type. The 

 author has not been able to obtain this in any case whatever ; but 

 the red type of the first series is well produceable and is the 

 first of a succession of diameters of fog particles, d^ = .00016, 

 d^ = .00032, c?3 = .00048, d^ = .00064, etc. The angular diameter 

 of d^ is about 60° showing the enormous size of the coronas in 

 question. The occurrence of the first series is corroborated by the 

 axial colors of the steam jet. 



4. Wilson's Conclusions os to Si::e and Numher of Fog Parti- 

 cles. — In the preceding section the conclusion was reached, that 

 the smallest corona-producing fog particles must exceed the order 

 of size, .0001 cm. Mr. C. T. R. Wilson believes that when " all 

 diffraction colors disappear and the fog appears white from all 

 points of view (adiabatic expansion 1.44) . . . the diameter of 

 the drops does not exceed one wave length of light, or 50 X lO"^ cm." 

 What Wilson refered to is probably the filmy disc of the red corona, 

 of the order of d^ = .00016 cm. It is therefore probable that Wil- 

 son's final greenish white color corresponded to about 10^ nuclei, or 

 that the filmy white implies two or three million nuclei, ^rather than 



