Chap. 2.] Light propelled in right Lines. - 179 



Venus in 1761 and 1769, the diameter of the earth's 

 orbit was found to be about 16^,636,800 geographi- 

 cal miles. When, therefore, the earth happens to be 

 on that fide of her orbit which is oppofite to Jupiter, 

 an eclipfe of his fatellites, or any other appearance in 

 that planet, is obferved to take place fifteen or fixteen 

 feconds later tha^ it would have done if the earth had 

 been on that fide of her orbit which is neareft to Ju- 

 piter *. From the very accurate obfervations of Dr. 

 Bradley, it appears that the light of the fun paflfes 

 from that luminary to the earth in eight minutes and 

 twelve feconds. 



The next property of light, to which it is proper 

 to advert, is, that it is detached from every luminous 

 or vifible body in all directions, and conftantly moves 

 in RIGHT LINES. It is evident that the particles of 

 light move continually in right lines, fmce they will 

 not pafs through a bended tube, and lince if a beam 

 of light is in part intercepted by any intervening body, 

 the fhadow of that body will be bounded by right 

 lines paffing from the luminous body, and meeting 

 the lines which terminate the interceding body: This 

 being granted, it is obvious, that the rays of light mud 

 be emitted from luminous bodies in every direction, 

 fmce, whatever may be the diftance at which a fpec- 

 tator is placed from any vifible object, every point of 

 the furface which is turned towards him is vifibie to 

 him, which could not be upon any other principle. 



The RARITY of light, and the minuteness of irs par- 

 ticles, are not lefs remarkable than its velocity. If 

 indeed the Creator had, not formed its particles infi- 

 nitely fmall, their exceflive velocity would be deftruc- 

 tive in the higheft degree. It was demcnftrated, that 



* See Newton's Optics, 1. ii. p. 3. prop, u, Prieftley's Op- 

 tics, p. 1 40. Nicholion's Phil. vol. i. p. 135. 



N 2 light 



