114 



KNOWLEDGE. 



[Mat 1, 1894. 



find no such hyperbolic strata enveloping the nucleus ; the 

 matter seems to be driven directly away from the nucleus, 

 and not in the first instance to be driven into an envelope 

 about the nucleus prior to its being driven away from the 

 sun. Perhaps the discrepancy may be explained by the 

 repulsion from the nucleus being comparatively feeble, so 

 that the envelopes about the nucleus are small and are 

 hidden by the nebulosity about the head of the comet. In 

 Swift's comet, the streams of matter radiating from the 

 nucleus all seem to be bounded by straight lines, though 

 they radiate in slightly different directions from the 

 nucleus. In the photograph of Brooks' comet there are 

 similar straight rays radiating from the nucleus in addition 

 to the curious curved and branching structure which 

 forms the brightest part of the tail. If we look for an 

 analogy with the more regular structures of Donati's comet, 

 it would seem that the great branching structure corre- 

 sponds to one of the jets which issued from the head of 

 Donati's comet, while the small straight rays in Brooks' 

 comet correspond to the streams which formed the bulk of 

 the tail in Donati's comet. 



The plate of Brooks' comet has been produced from a 



dense collodion enlargement, made from a glass positive 

 sent over by Prof. Barnard. It is entirely untouched, and 

 shows the great variations of brightness in different parts 

 of the tail. The spectral lines observed in the light from 

 the heads of different comets indicate that the chemical 

 constitution of one comet differs from another, and it is 

 perhaps to be expected that the general appearance of 

 comets, and the phenomena they exhibit, should also vary 

 according to the materials of which they are composed. 



The light from the remoter parts of the tail is usually 

 more or less completely polarized according to the position 

 of the 3omet with respect to the sun and earth, in a manner 

 which indicates that the matter of the comet's tail disperses 

 the sun's light as fine dust would do. It seems, therefore, 

 probable that the remoter parts of the tail of a comet 

 consist of very minute particles, whose average diameter 

 is small compared wdth the wave-length of light, and that 

 the material of which it is composed has been precipitated 

 from the gaseous matter driven off from the nucleus. 



In 1872 the medal of the Astronomical Society was 

 given to Prof. Schiaparelli, of Milan, for showing that 

 there was an intimate connection between certain meteor 



180° 



.lupiler's taiiiilv o< Comets 



270" 



The dotted portions represent the purts of the orbits below, i.e., to the eouth of the ecliptic. 



