412 Mr. W. Huggins and Dr. W. A. Miller on the 



accordant with those of the same lines taken in the solar 

 spectrum. 



Before commencing the examination of the spectrum of a star, 

 the alcohol-lamp was again placed before the object-glass of the 

 telescope, and the correct adjustment of the apparatus obtained 

 with certainty. The first observation was whether the star con- 

 tained a double line coincident with the sodium lines D. When 

 the presence of such a line had been satisfactorily determined, 

 we considered it sufficient in subsequent observations of the same 

 star to commence by ascertaining the exact agreement in posi- 

 tion of this known stellar line with the sodium lines D. 



Since from flexure of the parts of the spectrum-apparatus the 

 absolute reading of the micrometer might vary when the tele- 

 scope was directed to stars differing greatly in altitude, the mea- 

 sure of the line in the star which was known to be coincident 

 with that of sodium was always taken at the commencement and 

 at the end of each set of measures. The distances of the other 

 lines from this line, and not the readings of the micrometer, 

 were then finally registered as the measures of their position ; 

 and these form the numbers given in the Tables, from which 

 the diagrams of the star-spectra have been laid down. 



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 addi- 

 tional 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 sug- 

 gested 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 



* These measures, on repeated observation, seldom varied more than a 

 single division of the scale, or T sVo tn of the distance between A and H. 



