486 



PHYSICAL CONSTITUTION OF COMETS. 



from direct light, and the existence of which property there are sufficient opti- 

 cal means of detecting. He has, however, more recently furnished us with, 

 as we conceive, much more simple and satisfactory means of putting the ques- 

 tion finally at rest ; jf, indeed, it be not already decided. 



It is an established property of self-shining bodies, that at all distances from 

 the eye they have the same apparent splendor. Thus the sun, as seen from the 

 planet Herschel, seems as bright as when seen from the earth. It is true that 

 he is much smaller, but stiil equally bright. The smallest brilliant may be as 

 bright as the largest diamond. We must not here be understood to imply that 

 he affords the same light ; that is quite another effect. W'hat is intended to 

 be conveyed, will perhaps be best understood by considering the effect of 

 viewing the sun through a pin-hole made in a card. The card being placed at 

 a small distance from the eye, it is evident that the eye will view only a small 

 portion of the sun's disk, limited by the magnitude of the pin-hole ; but that 

 portion, so far as it goes, will be as bright as it would be were the card remov- 

 ed. Now, the effect here produced, by limiting the portion of the sun's disk 

 which the eye is permitted to see, is precisely the sam^ as if the eye were 

 carried to so great a distance from the sun, that its apparent magnitude would 

 be reduced to equality with that portion of its disk which is seen through the 

 hole in the card.* 



Now, applying this principle to the question of cometary light, it will follow 

 that, if a comet shines by light of its own, and not by light received from the 

 sun, it will, like all other self-luminous bodies, have the same apparent bright- 

 ness at all distances. It will, therefore, coase to be visible, not from want of 

 sufficient apparent brightness, but from want of sufficient visual magnitude. 

 Now, it may be shown that the limit of visual magnitude which would cause 

 the disappearance of a self-luminous body is so extreme, that it would be to- 

 ? tally inapplicable to this case. By varying the magnitude of the object-glass 

 ) of a telescope (which may be easily done), with which such a body is viewed, 

 ( in proportion to the magnifying power of the eye-glass, it is always possible to 

 make the image of the same apparent brightness ; that is, supposing the object 

 itself to maintain a uniform splendor. Consequently, if a body submitted to 

 this species of observation, cease to be visible even by a telescope, it will fol- 

 low, that it must disappear either by a very extreme diminution of visual mag- 

 nitude, or by the loss of its own intrinsic splendor. Now, to apply this test to 

 the question of comets. Let us ask in what manner they disappear ? Is their 

 disappearance the consequence of an excessive diminution of visual magnitude ? 

 or is it to be attributed to the diminished quantity of light which they transmit? 

 Every astronomer will immediately reply that the latter only can cause the 

 disappearance. The greater number of comets, including the most brilliant 

 and remarkable one of 1680 more especially, have obviously disappeared by 

 the gradual enfeeblement of their light. They were, as it were, extinguished. 

 At the very time they ceased to be visible, they possessed considerable visual 

 magnitude. But such a mode of disappearance is incompatible with the char- 

 acter of a self-luminous body, unless we suppose that, from some physical 

 cause, it gradually loses its luminosity. 



But in answer to this is adduced the observed fact, that the dimensions of 

 comets are enlarged as they recede from the sun ; that the luminous matter, 

 thus existing in a less condensed state, will shine with a proportionally enfee- 

 bled splendor ; and that at length, by the dilation of the body, the light be- 

 comes so dilute, that it is incapable of affecting the retina so as to produce 

 sensation. 



* This property is demonstrable by mathematical reasoning. 



