LIGHT. 



The colour of the animal creation, as well as of the 

 vegetable, depends on light. In the polar regions, ani- 

 mals and birds are generally white, while they become 

 more variegated as they approach the tropics. The inner 

 feathers of birds, to which the air has no access, are 

 generally white, and so are their bellies. The bellies of 

 fish are white for the same reason. Man deprived of 

 light soon droops, becomes sallow, dropsical, and dies. 



If light consists of particles of matter but this, not- 

 withstanding the opinion of Sir Isaac Newton, is far 

 from being generally entertained they must be most 

 minute, else, from the rapidity of their motion, they 

 would seriously affect the sight. They, however, do not 

 cause the slightest pain to the eye ; nor when condensed 

 by the most powerful burning glass, and thrown on the 

 most delicate balance, do they affect it, although they 

 would travel at the rate of nearly twelve millions of miles 

 in a minute. In illustration of the smallness of the par- 

 ticles of light, supposing light to be a material body, if a 

 candle be lighted, and so placed that no object shall ob- 

 struct its rays, they will be diffused over a space of more 

 than two miles in every direction, before the candle is 

 apparently diminished in the slightest degree. 



In reference to the velocity of light, it was thought for 

 a long period that light was propagated instantaneously ; 

 but from observations by Roemur on the eclipses of Ju- 

 piter's satellites, which regularly undergo occultations at 

 stated periods, it was found that they were visible about 

 sixteen minutes earlier when the earth was in that part 

 of its orbit nearest Jupiter, than when in the opposite 

 part ; showing that light is about sixteen minutes in pass- 

 ing across the diameter of the earth's orbit, a distance 

 of nearly 190 millions of miles. Circumstances of daily 

 occurrence tend to illustrate the velocity of light. The 

 flash of a gun is seen for a considerable time (if the dis- 

 tance is at all remote) before the report is heard ; as also 

 the lightning's flash before the thunder. 



The rays of light always proceed in straight lines, ex- 

 cept when they pass obliquely through media of different 

 densities. Those rays of light from the sun that fall per- 

 pendicularly on the atmosphere, pass in a straight line to 





