CHAPTER IV. 



THE NUCLEATION CONSTANTS OF CORONAS CONTINUED. 

 ON A METHOD FOR THE OBSERVATION OF CORONAS. 



48. Character of the method. In the usual practical experiments 

 with the large coronas of cloudy condensation (the largest types having 

 angular diameter of nearly 60), the source of light is placed in the 

 equatorial (vertical) plane of the fog chamber and remote from it. 

 The eye and goniometer are put as near it as possible whenever sharp 

 vision is essential. The diffracted rays in such cases come from the 

 fog particles at the ends of the chamber, as in fig. 24, a, and are liable 



FIG. 24. (a) Diffractions from fog particles at a, b, c, and a single source S, reaching 

 the eye at e. (6) Diffractions from fog particles at a, b, c, and two sources S', S", 

 with coronas n n' and n' n", in contract at n', reaching the eye at c. (c) Diagram 

 showing the relation of S, s', s, R, r, 6. (d) Case of two sources and coronas in con- 

 tact at n' drawn to scale. 



to be seriously distorted by the refraction of the glass walls. Further- 

 more, 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 measure- 

 ments. In fig. 24, a, with a source at 5 and an eye at e, the diffractions 

 of the fog particles a, b, c overlap. 

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