Wells, Wheeler and Penfield — IZiibidium, etc. 475 



through poor vacua and those of the solar corona, and for the 

 purpose of pointing out this similarity to others otherwise than 

 by verbal description, only, I resolved to photograph these 

 discharges under conditions similar to those under which the 

 solar corona is observed. Photographs 1, 2, 3, 4, 5, 6 are the 

 result. 



The discharges were obtained with the apparatus given in 

 fig. 10. The only additions were that a circular tinfoil disc 

 was pasted on the outside of the large bulb, in the line of sight 

 between the camera and the brass sphere a. The diameter of 

 this disc was about equal to that of the brass sphere. Also, 

 the inside surface of the large bulb which formed the back- 

 ground of the brass sphere was blackened by means of cam- 

 phor smoke to avoid reflections. The discharge in fig. 1 is 

 that of a good vacuum (about 2 mm ), the succeeding ones repre- 

 sent discharges in poorer vacua, the pressures varying between 

 2 mm and 60 mm . 



The bearing which these experimental results may have 

 upon the theory of the solar corona I prefer to leave to 

 others to decide. That they may prove a suggestive guide in 

 the study of solar phenomena seems not unreasonable to 

 expect. 



I am greatly indebted to Professor John K. Pees for the 

 interest which he took in my work, and to Mr. Mann of the 

 Columbia College Observatory, for the very valuable service 

 which he rendered me in photographing the coronoidal dis- 

 charges. 



Department of Electrical Engineering, Columbia College, 

 March 31st, 1892. 



Art. LX. — On the Hubidium and Potassium Trihalides ; 

 by H. L. Wells and H. L. Wheeler. With their Orys- 

 tallogra/phy ; by S. L. Penfield. 



The discovery of a series of caesium trihalides* has led the 

 writers to investigate the analogous rubidium and potassium 

 compounds. The following table gives a list of the bodies 

 which we have been able to prepare, together with the caesium 

 series for comparison. The compound KI . I 2 had been previ- 

 ously prepared by Johnson. f 



* On a Series of Caesium Trihalides, by II. L. Wells; including their Crystal- 

 lography, by S. L. Penfield. This Journal, III, xliii, 17. 

 t J. Chetn. Soc, 1877, 249. 



