20 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



CORONAS WITH MERCURY LIGHT. 



21. Preliminary survey. The inferences of the preceding papers* 

 gave the promise that on judiciously using monochromatic light as the 

 source of illumination the optical nature of the coronas might be fully 

 brought out. Such light must be strictly homogeneous and at the same 

 time very intense. Hence the usual methods of obtaining it are unsatis- 

 factory. The strong green line of a mercury lamp, however, fulfills the 

 requirements admirably, and this was therefore used. The results show 

 that the green disk and the first green ring alternately vanish as the result 

 of the interference phenomenon superimposed on the diffraction phe- 

 nomenon. If, therefore, the nucleation of a highly charged medium is sys- 

 tematically reduced, a series of angular diameters may be obtained, both 

 for the green disk and the inner or outer edge of the first green ring. 

 From the loci of these values the position of the first diffraction mini- 

 mum for green light may be inferred, and the size of droplets computed 

 from the usual equation for small opaque particles. 



If the reduction of the nucleation is accomplished by successive partial 

 exhaustions, all of them identical, while filtered air is allowed to enter 

 the receiver systematically between the exhaustions, the nucleations of 

 any two consecutive exhaustions should show a constant ratio. Allow- 

 ance must, however, be made for the subsidence during the later fogs and 

 for time losses, if any. This is the method used hitherto in my work and 

 the results seem to have been trustworthy. 



In the case of mercury light, however, it is now possible to compare 

 the latter with the former (diffraction) method of obtaining the diameter 

 of particles, with a view to throwing definite light on the optical phenome- 

 non. Subsidence methods are out of the question for large coronas, as 

 these are invariably fleeting in character and pass at once into smaller 

 coarse coronas. 



The results of the two methods may be regarded as coincident as long 

 as not more than 300,000 nuclei per cubic centimeter, or diameters of 

 particles not smaller than 0.0003 cm - are m question. For larger numbers 

 and smaller diameters the divergence rapidly increases. Indeed, for 

 particles larger than the size given, the optically measured loss per 

 exhaustion exceeds the exhaustion ratio, a result which is satisfactorily 

 explained by the contemporaneous subsidence of these relatively large 

 particles. For particles smaller than the limit in question, however, the 

 loss computed by the optic method is larger than the exhaustion loss, as 

 if fresh nuclei were produced or rather made available at each exhaustion. 

 It is this result which the present paper purposes to bring out in detail 

 and to consider in its bearings on the optical phenomenon. 



*Amer. Journ. Sci., xxv, 1908, p. 224; xxvi, 1908, p. 87; xxvi, 1908, p. 324. 



