SKCT. XXXVI. LIGHT OF COMETS. 357 



may come into view when the comet arrives at a darker 

 part of the sky, which were overpowered by the supe- 

 rior light of the sun while in his vicinity. The first of 

 these hypotheses he considers to be perfectly confirmed 

 by his observations on Halley's comet, made at the Cape 

 of Good Hope, after its return from the sun. He thinks 

 that in all probability the whole comet, except the dens- 

 est part of its nucleus, vanished and was reduced to a 

 transparent and invisible state during its passage at its 

 perihelion, for when it first came into view after leaving 

 the sun it had no tail, and its aspect was completely 

 changed. A parabolic envelop soon began to appear, 

 and increased so much and so rapidly that its augmenta- 

 tion was visible to the eye. This increase continued till 

 it became so large and so faint, that at last it vanished 

 entirely, leaving only the nucleus and a tail, which it had 

 again acquired, but which also vanished, so that at last 

 the nucleus alone remained. Not only the tails, but the 

 nebulous part of comets diminishes every time they re- 

 turn to their perihelia ; after frequent returns they ought 

 to lose it altogether, and present the appearance of a 

 fixed nucleus : this ought to happen sooner to comets of 

 short periods. M. de la Place supposes that the comet of 

 1682 must be approaching rapidly to that state. Should 

 the substances be altogether, or even to a great degree, 

 evaporated, the comet would disappear forever. Possi- 

 bly comets may have vanished from our view sooner than 

 they would otherwise have done from this cause. 



If comets shine by borrowed light, they ought, in 

 certain positions, 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 maybe altogether diaphanous, 

 others seem to possess a solid mass resembling a planet. 

 But whether they shine by their own or by reflected 



