NUCLEATION CONSTANTS OF CORONAS. 



On the other hand, it is the observational value of the aperture of the 

 given coronas which varies. Thus in fig. 26 the green coronas vary from 

 5 = 12 to 5 = 17 in the different series. Very probably mixed coronas 

 are being observed. To this must be added the subjective error or 

 personal equation which enters into the determination of contacts. 

 Finally, the tendency of a corona to shrink at once after the formation 

 of droplets makes it difficult to catch the time at which coronas should 

 be observed soon enough. Under other circumstances there is even 

 liable to be an oscillation of the coronal aperture in the lapse of time. 

 All these difficulties are accentuated as the coronas become larger, for 

 here not only are the droplets more volatile, but the coronas overlap, 

 and there is an unlooked-for tendency for them to flatten at the point 

 of contact. The dark rings are liable to invade the bright. 



The green coronas in table 34, series i and 2, and table 35, series 3, 

 show the following average values: 



Mean values are thus 



3 8.0 I O 6 <^ 3 = 400 5 2 =I4-3 IO 6 a 2 = : 



agreeing pretty well with the above data (Chapter III, section 33), where 



5 3 = 8.1 10^3 = 398 s 2 =i4.o io 6 ^ 2 228 



I may summarize, in conclusion, that the present section has developed 

 the method of observation by which data are obtained from the distance 

 apart of two sources of light when certain fiducial rings of the coronas 

 are put in contact. This method is superior to the above method with a 

 single source of light, although its full value has not been evidenced, 

 because of the extreme sensitiveness of the coronas to differences in the 

 distribution of the density of the nucleation. There is a second difficulty 

 inherent in the phenomenon itself, viz, the shrinkage or oscillation in 

 the size of coronas following the instant of their formation. It is prob- 

 able that the number of fog particles actually decreases by evapora- 

 tion, though the phenomenon is complicated by the coincident variation 

 of temperature. After relatively long waiting, a somewhat similar 

 shrinkage takes place throughout the period of subsidence, and in case 

 of large coronas the apparent nucleation may thus be reduced to one- 

 fifth of its original value. 



