40 S. P. Langley— Observations on Mount Etna. 
For all eye-studies of the photosphere, transparency is of 
so little value, that I have uniformly’ found direct telescopic 
study at Allegheny to be most successfully pursued on hazy 
; the very finest detinition I have ever obtained of the 
minute features of the photosphere and spots, being at times 
culty seen by four inches aperture at Palermo. 
I shall best give an idea of what may be expected ona moun- 
tain site, by direct extracts from my journal, 
“December 28th, 1878, 10 a.m. The air perfectly tranquil to 
the naked eye, and a calm so absolute that the sound of voices in 
conversation is heard at a distance of half a mile. 
* This extremely simple test of transparency is the best I know. 
