S. P. Langley— Observations on Mount Etna. 43 
High elevations have undoubtedly the advantage of dimin- 
ishing the atmospheric absorption of the more refrangible rays, 
an absorption so important that it probably cuts off from us the 
larger portion of the ultra-violet spectrum. . Cornu, whose 
work on this portion is so well known, informed me that he 
found himself able to add about 1 em. (on the seale of his 
map) to this ultra-violet end, for each 500 m. of altitude, and 
that it was his intention in order to extend his work further, 
to make the Furca pass in the Alps his observing station 
during the present year; a testimony of importance to the 
gain in this direction to be expected from mountain observy- 
atories. 
may say without reserve that for rapid progress, such observa- 
station. At an altitude of 10 or 11,000 feet we may still enjoy 
all the conditions of health which fit us for labor, but if we 
tions increasing very fast. If I may be allowed to quote from 
my own experience of a stay of ten days upon Pike’s Peak, at 
__A dry climate and a table land at an elevation of something 
like 10,000 feet, sheltered on the side of the prevalent winds 
e the most promising conditions in our present knowledge. 
Upon the whole, then, though the ideal station, where 
