74 C. Bar us — Coronas with Mercury Light. 



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. 



2. Apparatus. — The fog chamber was of the usual pattern, 

 cylindrical in form> with its axis horizontal. The clear walls 

 being of blown glass showed some refracting disturbances, not 

 however of serious character. The fog chamber was connected 

 with a large vacuum chamber by a short, wide passage way, 

 though width is of little consequence here. The cylinder was 

 lined with wet cloth, closely adhering, except at the narrow 

 horizontal windows for observation. 



For exhaustion this stopcock was suddenly opened at the 

 beginning of the first second, closed after five seconds and the 

 corona quickly measured. Filtered air was then at once intro- 

 duced and the next exhaustion made at the beginning of the 

 sixtieth second. This rhythm is essential. The isothermal 

 value of a drop of pressure [Sp^] was carefully predetermined. 

 It fixes the ratio, y, of the geometric progression of nucleation, 

 since 



y = (^-tt-^J) / (p-tt) 



where p is the barometric pressure and it the vapor pressure 

 at the given temperature. If the cock were left open for a 

 longer time than five seconds, [SjpJ would increase to the limit 



The goniometer was of the usual type, the eye being at the 

 center or apex, while two needles on radii 30 cm long registered 

 the angular diameter of the coronal discs or annuli. Formerly 

 the whole instrument was placed on the near side of a fog 

 chamber, the eye being about 30 cm from the nearest wall. 

 It conduces to much greater sharpness of vision, however, 

 and admits of a measurement of larger coronas, caet. par.. 

 if the eye is placed all but in contact with the nearer wall and 

 the needles (or in this case preferably the inner edges of round 

 rods) beyond the further wall. In such a case the refraction 

 errors are also diminished. In addition to these advantages 1 

 may mention the decidedly increased (about 25 per cent) value 

 of the aperture obtained. These excessive apertures show, 

 however, that the ordinary diffraction equation for coronas 

 is not fully applicable ; for aperture varies with the position 

 of the eye along the line of sight. It is often surprising how 

 large a corona can be measured by the second method in a 

 small fog chamber, scarcely six inches long. The distance 

 between lamp and chamber is kept about D = 250 cm . 



3. Equations. — The equations needed in the present work 

 are derived in my last Report,* and need merely be summarized 



* Carnegie Publications, No. 96, 1908, Chapters I, III (equat. 1 to 12). 



