C. Barus — Coronas with Mercury Light. 73 



Art. VI, — Coronas with Mercury Ligjit ; by C. Barus. 



1. Preliminary Survey. — The inferences of the preceding 

 papers* gave 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 

 homogenous and at the same time very intense. Hence the 

 usual methods of obtaining it are unsatisfactory. The strong 

 green line of a mercury lamp, however, fulfills the require- 

 ments admirably, and this was therefore used. The results 

 show that the green disc and the first green ring alternately 

 vanish as the result of the interference phenomenon superim- 

 posed on the diffraction phenomenon. If therefore the nucle- 

 ation of a highly charged medium is systematically reduced, 

 a series of angular diameters may be obtained both for the 

 green disc and the inner or outer edge of the first green ring. 

 From the loci of these values the position of the first diffrac- 

 tion minimum 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 succes- 

 sive 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. Allowances 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 diameters of particles, with a view to throwing 

 definite light on the optical phenomenon. 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 coinci- 

 dent as long as not more than 300,000 nuclei per cubic centi- 

 meter, or diameters of particles not smaller than -0003 cm are in 

 question. For larger numbers and smaller diameters the 

 divergence rapidly increases. Indeed, for particles larger than 

 the size given, the optical loss per exhaustion exceeds the 

 exhaustion ratio, a result which is satisfactorily explained by 

 the cotemporaneous subsidence of these relatively large par- 

 ticles. For particles smaller than the limit in question, the loss 



* This Jour., xxv, p. 224, 1908 ; xxvi, p. 87, 1908; xxvi, p. 324, 1908. 



