Light is received from the Heavenly Bodies. 23 



entire hemispherical surface at the same time. There must 

 be some one spot in it so strictly and accurately opposite to 

 the eye, that the straight line joining them, that is the course 

 of the ray from thence, must be truly perpendicular to the 

 surface of the hemisphere. But on every side of that point 

 such lines must i'ecede through all angular gradations, until 

 at the utmost limits of the visible surface they become tan- 

 gents to the sphere. In the course of this gradation, how- 

 ever, the rays are propagated, not merely at different angles, 

 but at increasing distances ; so that a ray leaving the circum- 

 ference of the visible surface must have a considerably further 

 space to traverse than one from its central point. We will 

 apply ourselves to that body from whence we derive almost 

 all our light, and consider for a moment what must be the 

 result in the case of the sun. That luminary, whatever may 

 be its substance, is a body of enormous size, having a diame- 

 ter which, although the measurements may vary in a slight 

 degree, must be somewhere about 800,000 miles, so that the 

 difference of distance between that traversed by a ray from 

 the nearest and from one of the extreme points of the visible 

 hemisphere, must be about 400,000 miles, the length of its 

 semidiameter. Now we know that light travels at the rate of 

 about 192,000 miles in a second; and it follows that a ray 

 from the circumference of the hemisphere must have received 

 its impulse more than two seconds earlier than one from the 

 superficial centre, in order that they may reach the eye at 

 the same time. 



It is difficult to suppose that so considerable a difference 

 both in space and time should not exert a certain influence 

 on some of the phaenomena of light within our observation. 

 In the first place, it is clear that the aberration, which is such 

 as to be a matter of necessary correction in observations from 

 this planet of the other heavenly bodies, must be greater by 

 this difference in the one case than in the other; and the ap- 

 parent place of the outer edge of the luminous hemisphere of 

 any one of them must be erroneous in proportion, as com- 

 pared with that of its superficial centre. In the next place, 

 since it appears that, as light requires time for its propaga- 

 tion, there must be some actual motion involved in its passage 

 from one jjoint to another, be its constitution what it may, it 

 is not unreasonable to suppose that the ultimate motion may 

 be somewhat more feeble in the case of the greater distance; 

 notwithstanding the extreme tenuity of its matter, which offers 

 to bodies moving through it no sensible resistance. That 

 ligfit does lose strength as it becomes further I'emoved from 

 its source, is unquestionable ; but this is for the most part an 



