64 Prof. Bartlett on the Comas and Tails of Comets. 
appear simultaneously, and §7¢, &c., reduce to zero, at equal 
intervals of which the duration is 
tr ate’ 
Ve x er X &e X &e. 
i= 
é sun, is, from the principle of wave propagation, much greater 
than in any other diredtlott ; and Herta the rae renter acta 
of the illumination on the side of the comet opposite the sun. _ 
The comet’s head can have no phases, from its self-luminosity; 
raged can La — — tail have sharply defined eb 
om the gradual degradation of molecu ions towards — 
shade bor Ex g o) —— perturbations to 
This view denies the presence of cometary material in the 
coma and tail altogether, and regards these appendages but a5 
phenomena due to the reciprocal action of the etherial and com 
etary molecular forces. According to it, the coma and tail be 
come, as it were, a luminous shadow, a part of which is literally — 
cast before, and the dark cap which envelopes the head and 
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