54 TRANSACTIONS OP THE [nOV. 25, 



coronal materials must be of inconceivable tenuity, since comets 

 cut their way through them without experiencing sensible re- 

 tardation. Not even Mr, Crookes' vacua can give an idea of the 

 rarefaction which this fact implies. Yet the observed luminous 

 effects may not in reality bear witness contradictory of it. One 

 solitary molecule in each cubic inch of space might, in Prof. 

 Young's opinion, produce them; while in the same volume of 

 ordinary air at the sea-level, the molecules number, according to 

 Stoney, 20,000 trillions."* It is to be hoped that Dr. Huggins' 

 method of photographing the corona without waiting for a total 

 eclipse may be carried to such perfection that in the future the 

 astronomers will be able to study this ''crown of glory'' more 

 comfortably and continuously. 



^ Gierke's " History of Astronomy during the Nineteenth Century," 

 p. 236. 



