252 ANNUAL OF SCIENTIFIC DtSCOVEUY. 



with the spectrum given by absolute sunlight, the difficulty of com- 

 paring the lines in the light reflected from a planet, visible only at 

 night with those in the light of the sun, visible only during the day, 

 had to be overcome. And this was accomplished by comparing the 

 sunlight reflected down from the atmosphere after sunset with the 

 light -of the planet when it first became visible. In this way, the lines 

 in the spectra of the different planets have been compared with the 

 principal lines of the solar spectrum, and with those produced in the 

 solar spectrum by the absorptive action of the earth's atmosphere. 



Observations on the Moon and the Planets. One of the first ques- 

 tions which presented itself to the minds of the observers, was to test 

 the existence or not of an atmosphere to our satellite, the moon. On 

 all astronomical grounds, the evidence of no appreciable atmosphere 

 is very strong ; but there are other points connected with the subject 

 which have a little contrary tendency. It is clear that the sun's light, 

 reflected from the lunar surface, must pass twice through the thick- 

 ness of any atmosphere the moon may have before reaching us, and 

 judging by the effects of the rays of the sun, when at a low altitude, 

 traversing a length of our own atmosphere, any lines due to its existence 

 should be readily seen. With this view the spectrum of the moon was 

 carefully examined on many occasions, and while it presented a perfect 

 accordance with the solar spectrum, and its principal lines, nothing could 

 be traced that was indicative of a gaseous envelop about the moon, and 

 the evidence of spectrum analysis may therefore be added to the other 

 reasons for believing that the moon, at any rate on the side presented 

 to our view, has little or no atmosphere. 



The planets, shining equally by light reflected from the sun, exhibit 

 numerous phenomena indicating the existence of atmosphere?, of which 

 the varying belts of Jupiter and Saturn, and the diminution of light at the 

 edges of these planets, and Mars, and Venus, may be mentioned. The 

 snow and ice of Mars also c mid only be produced by a gaseous envelop 

 like our own. The examination of the spectra of the planets, by Messrs. 

 Huggins and Miller, has however proved, in the first place, that the 

 planetary light, as has always been assumed by modern astronomers, is re- 

 flected light from the sun, the principal lines of the solar spectrum being 

 easily identified in the spectra of the planets ; and secondly, it has given 

 us some very interesting information respecting the existence and character 

 of the planetary atmospheres. Thus, in addition to the characteristic 

 lines of the sohr spectrum, there were observed in the planetary spectra, 

 various new lines, indicative of the presence, in greater or less quantity, 

 of substances which absorb certain lines of light. And these substances, 

 Messrs. Huggins and Miller are inclined to think, are indicative of dis- 

 tinctive features of the planetary atmospheres. It should here be stated 

 that the spectra of the planets are not easily rendered capable of exam- 

 ination. The li^ht of a planet consists of a portion of the rays radiated 

 from a disc, and a portion only of the light reflected from it therefore 

 passes through the slit of the spectroscope, and the spectrum is often ex- 

 ceedingly faint ; while the star, being a luminous point, the whole of its 

 light passes in to form the spectrum. 



In the case of the planet Jupiter, the light which comes to us from 

 this body was originally like the light which comes direct to us ; but 

 before it reaches us it has passed through the atmosphere of Jupiter 



