Color of the Air and of Deep Waters. 65 
AE, AF, the altitudes of the centre of the sun or astar, and the 
moon, corrected for dip and semi diameter, and their apparent dis- 
tance on BC. ‘The verniers a, 6, are fixed at the altitude and dis- 
tance, by means of the fixture screw—the two legs AE, AF, are 
opened until zero of the verniers a and 6 arein the same line with 
regard to the centre O; the screw S, is then turned, and the two legs 
AE, AF, are fixed at that opening—the two altitudes are then cor- 
rected for parallax and refraction-—(say the sun’s altitude has been 
set off on A E, the moon’s on A F,) move the vernier c to which m 
is hinged, down to the degrees, minutes and seconds, corresponding 
to the sun’s corrected altitude, and turn the fixture screw. Move a 
up to the moon’s altitude, corrected for parallax and refraction, and 
fix the vernier at it—then move the vernier 6 and turn the are BC 
on its hinge until zero of 6 stand in a line with zero on a, and the de- 
grees, minutes and seconds, upon which 6 stands, will be the true lu- 
nar distance from which the longitude is determined, as it is in all 
other methods. 
Arr. VII.—On the Color of the Air and of deep waters, and on 
some other Analogous Fugitive Colors; by Count Xavizrr De 
Maistre. 
Translated from the Bib. Univ. by Prof. J. Griscom. 
Tue blue color of the sky is accounted for, by supposing that the 
sun’s light reflected by the surface of the earth, is not entirely trans- 
mitted by the atmosphere and lost in space, but that the molecules 
of air reflect and disperse the blue ray. Why this ray is reflected in 
preference to the indigo and violet which are more refrangible and 
appear to be more easily reflected, is a circumstance not accounted 
for. 
The same blue reflexion is observed in deep sea water, and m 
lakes, and rivers, when they are limpid. 
The same singular phenomenon is also witnessed in various sub- 
stances of different natures which have no apparent analogy ; thus 
opaline substances are blue by reflexion: the noble opal, (indepen- 
dently of the partial rays which give so high a value to this stone and 
which are attributed to natural fissures*) reflects a general blue color 
* This was the opinion of the celebrated Haity- 
Vou. XXVI.—No. 1. 9 
