192 An Essay on a Property in Light 



coma, which are rays of light proceeding from the comet in the 

 same manner that the rays do from the sun ; with tliis difference, 

 that as the sun is intended to give hght and heat to other worlds, 

 his rays proceed in all directions, and consequently exhibit an 

 entire body of light wherever he is beheld (see Plate III. fig, 13), 

 while the comets (see fig. 12), as their light is intended only for 

 their own use, and as they do not need so much, send out their 

 light only in radii. 



These rays when they first proceed from the comet are close 

 together, and to a ceitain extent (the length of the coma), iu 

 consequence of being so tliick, send down a stronger light into 

 the eye than the secondary rays of the sun, and for that reason 

 are visible : beyond that point, these rays, by diverging as well as 

 diminishing, become as faint or fainter than the sun's ; and con- 

 sequently their light cannot be distinguished from his, except 

 that part of it which is directly behind the comet ; for, as the 

 sun's rays are intercepted by t!ie comet, the whole space which 

 the tail occupies, comparatively speaking, is in darkness, and 

 the tail is visible because there is no other light to drown it. 



If we suppose S (fig. 11) to be the sun, and C a comet, all 

 the ravs from the sun will be intercepted by the comet from the 

 space between the lines D D ; and in that space, but for the co- 

 met's rays, there would be total darkness; from D to T the sun's 

 rays are gradually admitted ; and his light gets stronger and 

 stronger till we come to T, where none of them are excluded. 



This figure so entirely corresponds with all the phienomena 

 relating to the comet, that I do not see how the principle can be 

 disputed : it is natural to expect that that part of the comet's 

 light which is contrasted with the greatest darkness, vvould be 

 the most conspicuous; and accordingly we find that the light of a 

 comet's tail is brightest between the lines D D, and fades away 

 gradually as it approaches tov/ards T. 



The shape of the comet's tail is marked out by the figure, and 

 its appearance exactly corresponds with the radii I have described 

 fig. 12. 



As the primary rays of the comets do not proceed in every di- 

 rection like those of the sun, but are merely radii, by the time 

 they reach the enrth they are so separated that not more than one 

 ray can enter the eye of the same person*. It is clear then that 

 all the light we see in the body of the comets is derived from the 

 Sim, and this will account for the phases they sometimes exhibit. 



The reason why the comets are not so brilliant as the planets, 



* Even to the inhabitant of the comet these rays will be so scanty that he 

 never will be able to distinguish them, even in the night-time, further than 

 as a faint light overspreading the retina, blended witli-the secondsuy rays 

 reflected into his eye at the sanae time. 



is 



