36 S. P. Langley— Observations on Mount Etna. 
vations, does not recognize steadily more than six stars in the 
Pleiades, and sees a seventh and eighth by glimpses, and on an 
ordinary clear night at Allegheny, I cannot steadily see the 
companion of Polaris with less than two inches aperture. 
I proceed to give some of the tests of light from observa- 
tions made on the nights of December 28th, 31st, 1878, and 
January Ist, 4th, 10th, 13th, 1879. The aperture employed is 
34 inches, where not otherwise stated. 
Pleiades with naked eye, moon not set, nine stars steadily 
isibl 
Companion of Polaris (position previously unknown), recog- 
nized with 1°6 in. aperture. 
Companion of Rigel steadily seen with 1°6 in. aperture, not 
seen with 1:4 in. 
eporis. Companion (mag. 11) seen with full aperture. 
Tauri. Companion (mag. 11°2) an easy object with full 
December 31st. The nebula in Orion being about 40° high and 
moon half full, the 5th star in the trapezium was seen with full 
z Orionis. The 3d star (11th mag.) seen with 3}in, A little 
later, however, the definition being worse, could not discern the 
11th mag. star preceding o Orionis. 
January 4th. 5th star in trapezium of Orion steadily seen, but 
could not be sure of 6th. 
3d star of 2 Orionis (11th mag.) seen with 3} in. in spite of 
moonlight. 
January 10th, 11th mag. star preceding o Orionis, also 11th 
mag. star of z Orionis well seen in spite of moon and the tremor 
from wind. 
The observations for stars included many others, which were 
less satisfactory, and I select from them this small list, with the 
remark that in the five nights which were all that presented 
themselves for this work, the wind was an almost constant 
obstacle to steady vision, while I was compelled to observe in 
the moonlight, and while from the construction of the tripod 
stand, I was generally obliged to omit objects at an altitude of 
much over 60°. Considering the distinctness with which 
objects of 11 to 11°2 m aitade .were seen under these circum- 
stances, I think we shall be justified in stating that the limit, 
for an ordinary eye and the aperture employed (3°25 in.), at 
this altitude on Etna cannot be far from the 115 magnitude of 
the Bedford catalogue, or the 10:2 magnitude of Struve’s scale. 
I have employed the scale of the Bedford catalogue partly be- 
cause Mr. Webb, in his very excellent little manual, makes th 
(in England), and because in the absence of more exact data, 
cea 
