which hither to has been unoL served ly Philosophers. 193 



is not, I imagine, because tliere is any thing peculiar eitlier in 

 themselves or their atmospheres, but because they are either 

 smaller than the planets ; or else, are further removed from us. 

 Mars is not so bright as Jupiter, although he is much nearer, be- 

 cause, as his bulk is smaller, and the sun's rays in consequence 

 diverge more from him than from Jupiter, he cannot send down 

 to us so great (compact) a body of light as Jupiter does. If a 

 beam of the sun's light be let into a dark room and strike upon 

 a concave mirror ; another beam upon a plain, and a third upon 

 a convex, mirror; and the persons in the room so disposed that 

 none of the direct rays will be reflected from cither into their 

 eyes; although I have never tried the experiment, I am fully per- 

 suaded that the. concave mirror will give out the greatest body of 

 ligbt, because the rays of light there will be more compressed 

 and close together ; and the convex, by the same rule, will give 

 out less than the plain mirror, because the rays striking upon it 

 diverge more, and are conset|uenily much thinner. If in the 

 place of the plain mirror we substitute a very large convex mirror, 

 the large mirror will give out a greater body of light from that 

 part of it on which the rays strike than the small, because its 

 surface approaches nearer to the plain, and its rays in conse- 

 quence do not diverge so much. Apply this rule to the heavenly 

 bodies, and we can easily account for their different degrees of 

 brightness. I have never seen any comet whose disk would mea- 

 sure so great an angle as that of Venus; and tlierefore they must 

 either have been further off, or their bulk must have been smaller; 

 and in either case they could not give out so great a body of 

 light. 



There is one other phaenomenon in the comet which I notice, 

 because it has excited the curiosity of a great many, although 

 my principle will not be any way affected whether I rightly ac- 

 count for it or not. The comet's tail sometimes, thougli not al- 

 ways, appears to be a little curved, andthe reason for it, in my 

 opinion, is this: When the comet is at a distance and going al- 

 most directly towards the sun, the direction of the tail does not 

 sensibly alter in the course of several hours, because the sun, 

 which is always opposite the tail, continues with respect to the 

 comet, to be very nearly in the same part of the heavens : but 

 when the comet is going round the sun, and has greatly increased 

 its velocity, every moment changes in a small degree the direc- 

 tion of the tail, and the impression of its rays docs not go off 

 instantaneously from the retina. Thus a burnt stick, waving 

 about, appears like a riband of fire, and the same cause may 

 also produce the apparent curve in a comet's tail. 



4. In order to account for the zodiacal light, it can hardly 



be necessary to say more than that 1 suj)posie it to be neither 



Vol. 55. No. 2G3. March 1820. N more; 



