380 LIGHT OF COMETS. SECT. XXXV. 



dark space between them, were formed in the head. From the 

 extremities of these the cone of the tail extended, and a non- 

 luminous or dark space stretched for 20 from the nucleus into 

 the tail. On the 1st October the two envelopes were combined 

 into one. This comet, like Halley's, has shown some singular 

 irregularities, supposed to arise from the action of the sun when 

 near its perihelion. At different periods of its apparition a 

 violent agitation was observed in its nucleus, with luminous jets, 

 spiral offshoots, &c., as in the great comets of 1680, 1744, 1811. 

 A ray of light was thrown out from one side of the nucleus 

 towards the sun, while a gas-like jet proceeded from the other 

 side, which appeared to form the origin of a second tail within the 

 great tail, and which was traced for half a degree by Mr. Hind 

 on the 19th September. He observed decided spiral convolu- 

 tions in the tail, which show that this comet has a rotatory 

 motion about an axis passing through the tail. 



If comets shine by borrowed light, they ought, in certain posi- 

 tions, to exhibit phases like the moon ; but no such appearance 

 has been detected, except in one instance, when they are said to 

 have been observed by Hevelius and La Hire, in the year 1682. 

 In general, the light of comets is dull that of the comet of 1811 

 was only equal to the tenth part of the light of the full moon 

 yet some have been brilliant enough to be visible in full daylight, 

 especially the comet of 1744, which was seen without a telescope 

 at one o'clock in the afternoon, while the sun was shining. 

 Hence it may be inferred that, although some comets may be 

 altogether diaphanous, others seem to possess a solid mass re- 

 sembling a planet. But whether they shine by their own or by 

 reflected light has never been satisfactorily made out till now. 

 Even if the light of a comet were polarized, it would not afford a 

 decisive test, since a body is capable of reflecting light, though it 

 shines by its own. M. Arago, however, has, with great inge- 

 nuity, discovered a method of ascertaining this point, independent 

 both of phases and polarization. 



Since the rays of light diverge from a luminous point, they 

 will be scattered over a greater space as the distance increases, so 

 that the intensity of the light on a screen two feet from the 

 object is four times less than at the distance of one foot ; three 

 feet from the object it is nine times less ; and so on, decreasing 

 in intensity as the square of the distance increases. As a self- 



