OF Tv'ATURAL H1STCHY. 1 S7 



diftance of the earth from the fun is fo immenfe, that, fup- 

 pofing a cannon ball to move at the rate of 500 feet in a 

 fecond, It could not come from the fun to the earth in lefs 

 than 25 years. At this rate, the velocity of light will be 

 above io million of times greater than that of a cannon ball. 



The rays of light, though they proceed in direct lines 

 from luminous bodies, are refra<Sled, or bent out of their courfe, 

 in paffi ng through different mediums, as the air, glafs, and 

 every tranfparent fubflance •, but, whenthey fall upon opaque 

 bodies, they are reflected. Rays proceeding from any 

 object, and pafling through a convex glafs or lens, are refract- 

 ed and colledled into a point, or fmall fpace, at a certain dif- 

 tance from the glafs, which is called the focus of that lens. 



The white light conveyed to us by the fun is not homoge- 

 neous, but confifts of feven differently coloured rays, or what 

 are called the primary colours. Thefe differently coloured 

 rays were difcovered by Sir Ifaac Newton to have different 

 degrees of refrangibiUty. When the white light of the fun 

 "vvas made to pafs through a glafs prifm, he found, that, in- 

 ftead of retaining its original whitenefs, it exhibited feven 

 diflindl: colours, and that this phaenomenon was produced by 

 the feveral rays in the compofition of white light being more 

 or lefs refra6led, or turned from their dire(^l courfe. The 

 iimple primary colours are {q\qi\ in number, namely, red, 

 orange, yellow, green, blue, indigo, and violet. Red is tl^e 

 leaft, and violet the moft refrangible parts of white light. 

 A proper mixture of all the feven primary colours confti- 

 tutes whitenefs ; and by various combinations of the prima- 

 ry colours, all the compound colours exhibited either in Na- 

 ture or art are produced. Any furface appears black when 

 it refle£ls little or no light. 



Tne different humours of the eye, and the cryftalline lens, 

 are all denfer than air or water ; of courfe, their power of 

 refra£ling the rays of light is likewife greater. The rays 

 proceeding from every point of an object enter the pupil l 



