THE SPECTEA OF SOME OF THE FIXED STAES. 
419 
The very close approximation*, not unfrequently the identity, of the measures 
obtained for the same line on different occasions, as well as the very exact agreement of 
the lines laid down from these measures with the stellar lines subsequently determined 
by a direct comparison with metallic lines the positions of which were known, have 
given the authors great confidence in the minute accuracy of the numbers and drawings 
which they have now the honour of laying before the Society. 
§ III. Observations on the Moon and Planets. 
7. It is well known that in the solar spectrum many additional remarkable lines 
make their appearance when light from the sun seen near the horizon reaches the 
observer, after having traversed a much greater length of our atmosphere than when the 
sun is viewed at greater altitudes. This circumstance suggested to us the importance 
of a careful examination of the solar light after reflexion from the moon and planets, in 
reference to the extent and analogous constitution of atmospheres possibly surrounding 
these bodies. As far as practicable, the spectra of the moon, Venus, Mars, Jupiter, 
and Saturn have been observed on several occasions with this special object in view. 
8. The Moon . — All the astronomical phenomena in which we should expect to dis- 
cover indications of an atmosphere about the moon, if such exist, agree in proving the 
non-existence of a lunar atmosphere of sensible amount. From the absence of appre- 
ciable refraction at the moon’s limb, and from the sudden extinction during a total 
lunar eclipse of stars of even the tenth and eleventh magnitude at the limb of the moon, 
“we are,” writes Sir John Herschel, “entitled to conclude that no amount of appre- 
ciable vapour is suspended near the surface of the moon, and the non-existence of 
an atmosphere at the moon’s edge having the 1980th part of the density of the earth’s 
atmosphere ”f. 
As by direct observation we know that the solar light is reflected from the surface of 
the moon, the light which reaches the earth after having undergone this reflexion must 
have passed through a length of lunar atmosphere, if such exist, at least equal to double 
the height of such atmosphere above that surface of the moon which is visible to us. 
From some parts of the moon, when the whole or a large part of its illuminated surface 
is turned towards the earth, the length of the column of lunar atmosphere which the 
solar light would have to traverse would be considerably greater. 
The examination of lunar light by the spectroscope, and the comparison of the light 
reflected from different portions of the moon’s illuminated surface with each other by 
this method, would take place under conditions favourable to the detection of an atmo- 
sphere of considerable extent, if such exist. 
The moon was examined by us on April 12 and November 26, 1862, March 31 and 
* These measures, on repeated observation, seldom varied more than a single division of the scale, or -j-g^-yth 
of the distance between A and H. 
f Outlines of Astronomy, 7th edition, par. 431, p. 284. See also a paper by Professor Chaxjjs in the 
Monthly Notices of the Eoy. Astron. Soc., vol. xxiii. p. 254, June 1863. 
