SCIENCE. 



335 



approximately 548.4. 



513.3, 467.2. The tail 

 gave no continuous 

 spectrum. The mean 

 of eighteen comets ob- 

 served gives us wave 

 lengths as follows : 



556.4, 512.7 and 470.6 ; 

 the mean of two 

 nights' work on this 

 comet gave me 549-3- 

 512.4 and 467.2. These 

 two sets of figures 

 agree as nearly as 

 could be expected, con- 

 sidering that I used in 

 my observations a 

 single 6o° prism, and 

 there can be no doubt 

 whatever that this is 

 the usual comet spec- 

 trum." 



Professor A. Hall 

 also observed that he 

 had received from 

 Baron Struve, of the 

 Imperial Observatory 

 at Palkova, an ephe- 

 meris of Encke's comet 

 extending from July 29 

 to November 14, and 

 preparations are mak- 

 ing at the Naval Ob- 

 servatory for careful 

 observations of that 

 hody, which is con- 

 sidered of great 

 scientific interest. 



We present our 

 readers with an il ? 

 lustration showing 

 the appearance of 

 the comet in one of 

 Professor Henry 

 Draper ' s photo- 

 graphs for which we 

 are indebted to 

 Messrs. Harper 

 Brothers, and we 

 hope to shortly pub- 

 lish Professor Drap- 

 er's mature views 

 based on his obser- 

 vations and photo- 

 graphs, both of the 

 comet and its spec- 

 trum. 



To a correspond- 

 ent Prof. Draper 

 gave the following 

 particulars : 



" In the spectrum 

 of the comet there 

 is one great band 

 in the ultra-violet re- 

 gion beyond the line 

 H. This morning 

 I brought the spec- 

 troscope with me to 

 the city, and have 

 taken photographs 

 of the spectrum of 

 the electric arc with 

 it. The electric arc 



Group of Various Comets. 



contains carbon ; not, 

 in all probability, the 

 pure element of carbon 

 volatilized, but some 

 compound of carbon — 

 most likely a hydro- 

 carbon. The spectrum 

 of the electric or voltaic 

 arc shows a strong 

 band at the ultra-vio- 

 let region, due to the 

 presence of this carbon 

 or carbon-compound. 

 This spectrum I mean 

 to compare with that 

 of the comet, to see 

 whether the bands in 

 the ultra-violet region 

 correspond in the two. 

 If they do, the pres- 

 ence of some form of 

 carbon in the comet 

 will be demonstrated. 

 My impression at pres- 

 ent is that the ultra- 

 violet spectrum of the 

 comet does prove that 

 it contains carbon, but 

 I cannot speak with 

 certainty until after I 

 have made more care- 

 ful measurements of 

 the photographs. 

 At all events, my 

 experiments must 

 settle the question. 



We are indebted 

 to Professor Picker- 

 ing, of Harvard Uni- 

 versity, for some 

 valuable observa- 

 tions which will be 

 found in another col- 

 umn. 



Views of Encke's Comet, 1871 



ON THE TAILS 

 OF COMETS. 



One of the most 

 important articles in 

 the June number of 

 Urania is that by 

 M. Th. Bredichin, 

 on the Tails of Com- 

 ets. After a series 

 of investigations he 

 arrives at the con- 

 clusion that the po- 

 sition, curvature and 

 structure of a tail 

 are explained by the 

 repulsive force of the 

 sun and by the efflu- 

 via of cometary mat- 

 ter from the nucleus 

 towards the sun with 

 a certain initial ve- 

 locity or repulsion. 



