NUCLEATION CONSTANTS OF CORONAS. 77 



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 fig. 24, b, and to observe the contact in this plane 

 of the two identical coronas produced. In this way the oblique refrac- 

 tions are diminished as far as possible, coronas of all sizes are observable, 

 and both eyes are available for observation, increasing sharpness of vision 

 and lessening the eye strain. The contact method is in itself more 

 sensitive, seeing that the eyes may be placed all but in contact with the 

 fog chamber. In fig. 24, b, with two sources at 5' and S" and the coronas 

 nn' and n'n" in contact at n' at the edge of a given annulus, the diffrac- 

 tions of the fog particles a, b, c overlap. 



49. Apparatus. Fig. 24, d, shows a general disposition of the appa- 

 ratus. S' and 5" are the two circular sources of light lying in the same 

 horizontal, and movable in opposite directions in equal amounts, at the 

 control of the observer at the fog chamber F. S' and 5" are therefore 

 always symmetrical with respect to the vertical plane SR. The diffrac- 

 tion of rays due to the fog particles in F produces coronas seen at nn' and 

 n'n", and the lamps S'S" have been adjusted at a distance 5, so that 

 the selected annuli of the coronas are in contact at n'. The angular 

 radii of the coronas, marked 6 or shaded in the diagram, are nearly 

 equal and 2R tan 6 = 5, where R is the distance of the axis of the fog 

 chamber from the track 5. 



On a double track, at 5, the two carriages for the lamps S'S" are 

 moved with sprocket and chain or in a similar manner, and provided 

 with a scale stretched between them, reading to centimeters. This scale 

 is a lath of wood about 3 meters long, with one end fastened at S', the 

 other free, while the scale moves across an index at 5". A pole at R, with 

 the end in the observer's hand, moves the two central sprockets and at 

 the same time serves for the measurement of R, should this vary. 



50. Errors. Fig. 24 shows clearly that the angle of diffraction cor- 

 responding to the fog particles a, b, c, nearer and farther from the eye, 

 will not be the same, and that this effect will vanish as the coronas are 

 smaller, as the diameter or thickness of the fog chamber is less, and as 

 the distance R from the source is greater. Slightly different annuli 

 overlap; but the effect is much less here than in the case of a single 

 source, where the active fog particles lie oblique to the axis. (See fig. 

 24, a, and fig. 24, b, at a, b, c.) In practice this effect is probably negligible 

 if the dimensions of apparatus and disposition of parts are properly 

 chosen, particularly so since the fog particles themselves are not usually 

 so nearly of a size as to imply less overlapping. In fact the true corona, 

 if large or even of moderate size, is seen but for an instant immediately 

 after exhaustion. It thereafter shrinks rapidly, as may be gathered from 



