302 SCIENTIFIC RECORD FOR 1682. 



for the comet many times before sunrise and after sunset, but without 

 detecting it. 



The comet has been sought for elsewhere, though unfortunately with- 

 out success. 



The refractive power of comets. — The great comet 1881, III, passed 

 near to faint stars on several occasions, so near that the stars were 

 certainly shining through the cometic substance. During three such 

 opportunities, Dr. Meyer, at the Geneva Observatory, made a series of 

 micrometer measures on the relative positions of the star and the 

 comet's nucleus. This last was sharp and well defined like a star. 



The preliminary reduction of these measures showed that the light 

 of the star suffered a refraction in its passage through the comet's head. 

 The whole question of such a refraction was examined by M. Cellerier 

 (Archives des Sciences, etc., de Geneve, October, 1882) under the suj)- 

 position of a variable density to the cometic matter, which was further 

 supposed to act like a true gas. The theoretical solution of the prob- 

 lem is given in a simple form. Dr. Meyer has a^iplied the theory to the 

 cases in hand, and obtains from the three stars the following values of 

 e, which is the refracti%"e power of the cometic matter (supposed gaseous): 



29 June, 1882, e = 0.0000091G; 14".3 = di. 



13 July, 1882, e = 0.0((000299; 25".3 = d^. 



1 August, 1882, e = 0.00000317 ; 24".G = <?3. 



The numbers <?i, ^25 ^2, are the shortest distances of the stars from 

 the comet's nucleus, reduced to what they would have been if seen all 

 from a distance 1. Sujiposing the density of the gas to be proportional 

 to the square of the distance, and the refractive power of the gas to be 

 directly proportional to the density, the three values of e abeve can be 

 reduced to the following three numbers which express (on this hypoth- 

 esis) three different determinations of the refractive power of the gay 

 at a distance of 14".3 = 102,000 kilometers. 



29 June, eo = 0.0000091G. 



13 July, =0.00000936. 



1 August, = 0.00000938. 



Dr. Meyer epitomizes his results as follows: 



"The substance which composed the head of comet 1881, III, behaved 

 optically like a gas, and its refractive power at a distance of 102,000 

 kilometers from the head was 0.0000093. This refractive power, and 

 hence the density of the gas itself, varied as the square of the distance 

 from the comet's nucleus." 



Professor Bredichin has issued the first part of Vol. viii of 7l?tMa?es 

 de r Observatoire de Moscoii, which, in addition to meridian observations, 

 contains a continuation of his researches upon the tails of comets, the 

 present publication including the comets 1881 h and c, and the fourth 

 or great comet of 1825. Professor Bredichin has reprinted the long 

 series of physical observations on the latter body made by Dunlop at 



