414 



Lord Rayleigh. On the Dark Plane [Dec. 21, 



Postscript. — Received December 15, 1882. 



I liave Captain Abney's permission to add the following letter this 

 day received from him. " A careful examination of your series of 

 .sun-photographs, taken with absorbing media, convinces me that yonr 

 claim to having secured photographs of the corona with an nneclipsed 

 sun, is fully established. A comparison of your photographs with 

 those obtained during the eclipse which took place in May last, shows 

 not only that the general features are the same, but also that details, 

 such as rifts and streamers, have the same position and form. If in 

 your case the coronal appearances be due to instrumental causes, I 

 take it that the eclipse photographs are equally untrustworthy, and 

 that my lens and your reflector have the same optical defects, j 

 think that evidence by means of photography of the existence of a 

 -corona at all is as clearly shown in the one case as in the other." — 

 Deo. 15, 1882.] 



IV. " On the Dark Plane which is formed over a Heated Wire 

 in Dusty Air." By Lord RAYLEIGH, F.R.S., Professor of 

 Experimental Physics in the University of Cambridge. 

 Received December 8, 1882. 



In the course of his examination of atmospheric dust as rendered 

 .evident by a convergent beam from the electric arc, Professor Tyndall 

 noticed the formation of streams of dust-free air rising from the 

 summits of moderately heated solid bodies.* " To study this effect a 

 platinum wire was stretched across the beam, the two ends of the 

 wire being connected with the two poles of a galvanic battery. To 

 regulate the strength of the current a rheostat was placed in the 

 circuit. Beginning with a feeble current, the temperature of the wire 

 was gradually augmented ; but before it reached the heat of ignition, 

 a flat stream of air rose from it, which, when looked at edgeways, 

 appeared darker and sharper than one of the blackest lines of Fraun- 

 hofer in the solar spectrum. Right and left of this dark vertical band 

 the floating matter rose upwards, bounding definitely the non-luminous 

 stream of air." .... 



"When the wire is white hot, it sends up a band of intense dark- 

 ness. This, I say, is due to the destruction of the floating matter. But 

 even when its temperature does not exceed that of boiling water, the 

 wire produces a dark ascending current. This, I say, is due to the 

 .distribution of the floating matter. Imagine the wire clasped by the 

 mote-filled air. My idea is that it heats the air and lightens it, without 



* " Proc. Eoy. Inst,," vol. 6, p. 3, 1870. 



