C. Barus — Method for the Observation of Coronas. 277 



Akt. XXVIII. — On a Method for the Observation of 

 Coronas*; by C. Barus. 



1. Character of the method. — In the usual practical experi- 

 ments with, the large coronas of cloudy condensation (the 

 largest types having angular diameter of nearly 60°), the small 

 round source of light is placed in the equatorial (vertical) plane 

 of the fog chamber and remote from it. The eye and goniom- 

 eter are put as near it as possible, whenever sharp vision is 

 essential. The diffracted rays in such cases come from the fog 

 j>articles a, b, c, at the ends of the chamber F, as in figure la, 

 and are liable to be seriously distorted by the refraction of the 





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glass walls. The limit will be reached sooner or later, in which 

 the fog particles to which the diffractions are due lie at or 

 beyond the ends of the fog chamber, after which the features 

 essential to the measurement will no longer appear. Moreover 

 one eye only can be used in the measurements. 



It occurred to me, therefore, to invert the phenomenon by 

 using two sources, which may be moved symmetrically towards 

 or from the equatorial plane, as in figure lb, and to observe 

 the contact in this plane of the two identical coronas produced. 

 In this way the oblique refractions are diminished as far as 

 possible, the diffracting particles a, b, c lie in the middle, 

 coronas of all sizes are observable, both eyes are available for 

 observation, increasing sharpness of vision and lessening the 

 eye strain. The contact method is in itself more sensitive, 



* Extract from a forthcoming report of the Carnegie Institution of 

 Washington. 



