106 On the Formation of the Tails of Comets. 



the whole nucleus is composed of the same species of nebulous 

 matter as the other portions of the comet. 



The tail, though generally less bright than the nebulosity of 

 the head, is not always so ; e. g. the comet of 1843, as seen on 

 the 28th of February.* 



Whether comets are self-luminous or shine by reflected light, 

 is a question which, though it has been much discussed, is not 

 yet settled. Since the evidence of a reflection of light, sought 

 for in phases of the nucleus and in signs of polarization in the 

 light received from a comet, has not been found sufficient to 

 establish its existence, Arago proposes another method of research 

 in the case, founded upon observed variations in the intensity of 

 the light, which may perhaps be more successful. 



2. The structure of Comets. — A cometary body is composed 

 of two connected portions of nebulous matter, called, respectively, 

 the head and the tail. The head consists, in general, of a central 

 nucleus, (although having a greater lustre, composed as we have 

 seen reason to believe, of the same species of matter as the other 

 parts, but in a more condensed state.) enveloped on the side towards 

 the sun, and ordinarily at a greater distance from its surface in 

 comparison with its own dimensions, by a globular nebulous 

 mass, called the nebulosity. This, it is said, never completely 

 surrounds the nucleus, except in the case of comets which have 

 no tails. It in fact encircles only that hemisphere of the nucleus 

 which is turned towards the sun. The tail begins where the nebu- 

 losity terminates, and seems, in general, to be merely the continu- 

 ation of this in nearly a straight line beyond the nucleus. There 

 is, ordinarily, a distinct space containing but little luminous mat- 

 ter between the nucleus and the nebulosity, but this is not always 

 the case. From the accounts that have been given of the telesco- 

 pic appearances of the comet of 1843, it would seem that the nu- 

 cleus and nebulosity of this comet were seldom visibly separate; 

 what is there called the nucleus, being in fact the nebulosity and 

 nucleus combined. The tail of a comet has the shape of a hol- 

 low frustum of a cone, with its smaller base in the nebulosity of 

 the head ; with this difference, however, that the sides are usu- 

 ally more or less curved, instead of being straight. They are, in 

 general, concave towards each other, as in the case of the recent 



* See American Almanac for 1844, p. 94. 



