298 WELLS'S NATUEAL PHILOSOPHY. 



paper, a few feet apart on a wall, and throw the light of one candle on one 

 disc, and the light of the other candle upon the other disc. If they are of 

 unequal illuminating power, the candle which affords the most hght must 

 be moved back until the two discs are equally iOuminated. Then, by meas- 

 uring the distance between each candle and the disc it illuminates, the lum- 

 inous intensities of the two candles may be calculated, their relative intensi- 

 ties being as the squares of their distances from the illuminated discs. If, 

 when the discs are equally illuminated, the distance from one candle to its 

 disc is double the distance of the other candle from its disc, then the first 

 candle is four times more luminous than the second ; if the distance be triple, 

 it is nine times more luminous, and so on. 



Instniments called "Photometers," operating in a similar manner, have also 

 been constructed for measuring the relative intensity of two luminous bodies. 

 Their arrangement and plan of operation is substantially the same as in tho 

 method described. 



648. The light of the sun greatly exceeds iu 

 most intense iiitensitv that deiived from any other lumin- 



Ught known? , ; *' 



ous body. 



The most brilliant artificial lights yet produced, are very far inferior to tho 

 Bplendor of the solar liglit, and when placed between the disc of the sun and 

 the eye of the observer, appear as black spots. 



Dr. Wollaston has calculated that it would require twenty thousand mil- 

 lions of the brightest stars like Sirius to equal the light of the sun, or that 

 that orb must be one hundred and forty thousand times further from us than 

 he is at present, to bo reduced to the illuminating power of Sirius. 



The light of the full moon has also been estimated as three hundred thou- 

 sand times less intense than that of the sun. 



During the day the intensity of the sun's light is so great as to entirely eclipse 

 that of the stars, and render them invisible ; and for the same reason, we only 

 notice the light emitted by fire-flies and phosphorescent bodies in the dark. 



Are the more- ^^9. Light docs Hot pass instantancously 

 Sst"antancofs? through space, but requires for its passage from 



one point to another a certain interval of time. 

 With what re- The velocity of light is at the rate of about 

 trarrir"^^ ^ One hundred and ninety-two thousand miles in 



a second of time. 



.„^ , ., Light occupies about eight minutes in traveling from the 



What are il- ° ^ ° ^ 



lustrations of sun to tlie earth. To pass, however, from the planet 



r'-'ht^?'^"''''^ "^ Uranus to tho earth, it would require an interval of three 



hours. 



Tlie time required for light to traverse the space intervening between the 



nearest fixed star and the earth, has been estimated at SJyears ; and from 



the farthest nebulae, a period of ssveral hundred years would be requisite, so 



