256 LOWELL— REPULSION OF GASEOUS -MOLECULES [April 21, 

 Tail of Hallev's Comet. 



Angular Distance from the Nucleus to 

 the Point Measured in the Tail. 



Velocity of the Point of the Tail Away 

 from the Nucleus. 



13.6 miles a second 

 17.2 " " " . 



19.7 " " " 

 29.7 " " " 



3. \\'hile the series of direct photographs was being taken two 

 series of spectrograms were being carried on by Dr. SHpher, one with 

 an objective prism; the other set through a sht. The objective prism 

 ones recorded simultaneously the spectrum of the nucleus and head, 

 together with that of the tail out to about 11° from the nucleus. 

 One of them was got on Alay 23 at the same time as the photo- 

 graphs measured; while others were obtained on dates before and 

 after. Of the direct information afforded by these spectrograms of 

 the constitution of the comet an account is given in the extensive 

 monograph on the comet published by the Lowell Observatory. 



4. But a third result was obtained by the unwitting collaboration 

 of the spectrograms and the photographs. While the photographs 

 were giving their pictures of the tail, the objective prism spectro- 

 grams were doing the like, with this difference that they recorded in 

 a row pictures of it in the several colors of the spectrum, sifting 

 out into a band those made by each separate wave-length of light. 

 They thus made it possible to tell to what wave-lengths the visible 

 appearances were due. For it became evident at once from the 

 spectrograms that all wave-lengths were not equally concerned. On 

 the contrary, there were in the spectral image several distinct tails 

 with spectral gaps between. By an analysis of the wave-lengths 

 yielding pictures of the tail was thus offered a diagnosis of the sub- 

 stances composing it. In this way it appeared that CO.,, carbon 

 monoxide, was the chief constituent of the tail ; CH^, marsh gas, 

 another; CX, cyanogen, a third component; and minute solid parti- 

 cles, giving a more or less continuous spectrum, a fourth. 



That not one but a series of spectrograms was taken was impor- 

 tant. It not only gave us a constitutional history of the tail but it 

 showed the necessity of simultaneity in photographic and spectro- 

 graphic observations for comparative purposes. For the series 



