S. P. Langley— Observations on Mount Etna. 39 
Reviewing my experience, I should say that the gain on 
Etna over a lower station, as tried by the tests of a double-star 
observer, was more in clearness of the atmosphere than in that 
freedom from tremor which accompanies good definition. The 
latter was indeed upon the whole better than below, but not 
conspicuously so. 
had occasion in August, 1878, to notice the remarkable 
extent and brightness of the milky way, as seen from the Colo- 
rado plains, and still more from Pike’s Peak. The appearance 
of a nebula is perhaps indeed the best test to an experienced 
eye, of the quality of transparency in any atmosphere, and I 
was desirous of making a sketch of the nebula of Orion, as a 
useful measure of this transparency at my station. It was 
not easy to do this, however, as in most of the few clear 
hours which presented themselves, I was troubled by moon- 
light. The whole time I was thus able to give the sketch 
was hardly equal to more than two or perhaps three consec- 
utive hours, although something was done at it on every 
opportunity, when other work permitted. With undivided 
attention [ believe many more details might have been 
gathered. 
drawn ; but this is not the only cause. In observations of 
precision, we generally seek before everything an optically 
_* The drawings made are not reproduced here, but the originals are at the ser- 
vice of any one desiring to institute a comparison under like conditions.—-s. P. L. 
