50 Mr. S. M. Drach on deducing the Parallax of Mars, 



In the absorption 



In the flame 



spectrum. 



spectrum. 



72 



70—71 



73—74 



72—73 



75—76 



74—75 



77—78 



76—77 



79—80 



78 



81 



82 



83 



84 



85 



86 



86—87 Fraunh. 



D 88 



88—89 





90—91 



91—92 



92—93 



93—94 



94—95 



95—96 



96 





The conclusions of the absorption theory are thus seen to be 



confirmed in the most beautiful manner by the spectrum of in- 



candescent iodine gas : incandescent iodine just emits that light 



which the violet vapours of iodine absorb. 



I did not succeed in obtaining 



this agreement in the case of 



bromine vapour ; for the light of its flame was too feeble to give 



its spectrum in a state of purity. 





VIII. On deducing the Parallax of Mars, and thence that of the 

 Sun, from the former's Geocentric Velocity when in opposition, 

 and especially near the Node, By S. M. Drach, F.R.A.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 V17'ITH the above title I presented in 1843 a paper to the 



▼ t Royal Astronomical Society (vide Monthly Notices, 

 vol. ix. Jan. 1844) ; and as a similar mode of finding the true 

 value of the sun's parallax has lately and successfully been prac- 

 tised, I beg to communicate, through your Journal, the complete 

 paper, only a short extract of which was at the time inserted in 

 the ' Monthly Notices ' of the Royal Astronomical Society. 



" The transits of Mercury and Venus have hitherto been deemed 

 the only accurate means of ascertaining the small quantity of 

 solar parallax, by observing the time employed by those planets 

 in traversing the sun's disc. The effect of parallax, though 

 hardly measurable with exactness in space, was here so much 

 magnified that (vide 0. Gregory's ' Astronomy/ p. 385) the ob- 

 served differences of duration at Otaheite andWardhuus amounted 

 to 23 m 10 s , which considerable interval, properly applied to the 



