L I G 



4. According to Leslie, in passing through sea water, light is diminish- 

 ed four times for every five fathoms of vertical descent; and Bouguer 

 asserts, that the whole effect of the sun's light would be lost by passing 

 through b'79 feet of sea water, and that the same effect would take place 

 by its passing through 3,110,310 feet of air. 



5. Bouguer computes that of 300,000 rays which the moon receives ; 

 172,000, or perhaps 204,100 are absorbed ; and that the light of the sun : 

 ditto of the full moon :: 300,000 : 1. 



6. Euler makes the light of the sun equal to that of 6560 candles at one 

 foot distance ; that of the moon to a candle at 7| feet ; of Venus to a can- 

 dle at 421 feet ; and of Jupiter to a candle at 1620 feet, partly from Bou- 

 guer's experiments. Hence the sun would appear like Jupiter, if re- 

 moved to 131,000 times his present distance (Young's Nat. Phil.} 



Light, refrangibility of. 



7. The sun's light consists of rays which differ in refrangibility and 

 colour. 



The 1 primary colours are red, orange, yellow, green, blue, indigo, and 

 violet, of which the red rays are the least refrangible, and the violet ones 

 the most ; while green and blue are the colours which have a mean de- 

 gree of refrangibility. Sir Isaac Newton found their degrees of re- 

 frangibility in passing out of glass into air to be as the numbers 77, 



77JL > 77J_ } 77-L, 77__ 9 77 77 ' and 78, those being the values of the 



8 o 3 2 3 " 



sines of refraction to the common sine of incidence 50. Some substances, 

 however, separate the different coloured rays more widely than others, 

 and the dispersive power of media does not appear to depend at all upon 

 their mean refracting power. 



To find a measure of the dispersing power, take a constant small / 6 

 for the / of refraction, the / of incidence will then be m and will dif- 

 fer according to the value of m. The difference between these two or 

 (m- 1) 6 is the refraction; and if m and m be values of m for led and 



r v 



violet rays, the difference of refraction will be (m 1 ) 8 -~ (7/1 1} 5 or 



v r 



m m 



(m . m . 0. Its ratio to the refraction will consequently be 



v r m ' 



taking the mean value of m '. this is the usual measure of the dispers* 



iug power. 



In flint glass its value is about 0.05; in crown glass 0,033. 

 t Having- giveu th refracting- power* of two mediums, to find the 



