1882.] on Comets, .3 



2. The coma. This appears usually as a luminous fog surrounding 

 the nucleus, and gradually shading off from it. The nucleus and 

 the coma form together the head of the comet. 



3. The tail. The tail may be considered as a continuation, in a 

 direction opposite to that of the sun, of the luminous fog of the coma. 

 This appendage may be scarcely distinguishable as a slight elongation 

 of the coma, or it may extend half across the heavens, and be many 

 millions of miles in length. The tail may be single, or composed of 

 several branches. 



We must now study more closely the cometary ap^Dearances as 

 they may be seen when a large telescope is directed to a brilliant 

 comet. I have selected for this jDurpose the Great Comet of 1858, 

 and I shall exhibit on the screen a series of views of this comet, taken 

 at intervals of a few days. The first set shows the growth, position, 

 and forms of the tail, as a whole. The second group represents the 

 more detailed structure and changes of form of the head of the comet, 

 as viewed in a large telescope. These views are, of course, from 

 sketches made at the telescope. Last year several attempts were 

 made to photograph the comet which appeared in June. Mr. Janssen 

 has kindly sent me a positive taken from the original negative. It 

 is now upon the screen. Mr. Janssen purjDosely sacrificed detail in 

 the head of the comet, for the sake of obtaining the structure and 

 form of the tail, exjDOsing the plate for thirty minutes. From a 

 careful examination of several similar negatives, Janssen made a 

 drawing of the comet. A j)hotograph of this drawing is now upon 

 the screen. Mr. Common, at Ealing, with a fine three-foot reflector 

 of his own construction, also photographed the comet, but his object, 

 different from that of Janssen, was to get the form of the nucleus. 

 For this purpose he gave an exposure of only ten minutes — far too 

 short to obtain an impression of the tail. The comet was also 

 photographed by Dr. Draper, of New York. My own work was 

 confined to the comet's spectrum, of which I shall S2)eak presently. 



We may now advance to the consideration of two primary ques- 

 tions : — 



1. Does a comet shine wholly by reflected solar light, or has it 

 also light of its own? 



2. Of what materials is a comet composed ? 



The spectroscope has furnished us with information on both these 

 points. The first successful application of the spectroscope to a comet 

 was in 1864, when Donati discovered in its light three bright bands. 

 In 1866 I was able to distinguish two kinds of light from a telescopic 

 comet — the one kind giving a continuous spectrum and presumably 

 solar light, and the other a spectrum of three bright bands, similar 

 to those which had been seen by Donati. But in 1868 a great advance 

 was made. The close agreement of measures I took of the bands of 

 the comet h of that year with those I had previously taken of the 

 spectrum of certain compounds of carbon led me to compare, directly, 

 in conjunction with my friend Dr. W. Allen Miller, the spectrum of 



B 2 



