OnlAghi. 415 



multiplied by 0*45302, the absolute refractive power of air, when 

 we take the density of water for unity, we shall have a product 

 = 0'67984. Now, according to Newton's estimate, which M. Biot 

 has found to be exact, the refractive power of water is 0*7 845. 

 Hence we see that the compound has acquired an increased re- 

 fractive force by condensation, above the mean of its constitu- 

 ents, in the ratio of 100 to 86f . 



Rays of light, in traversing the greater number of crystallized 

 bodies, are commonly split into two pencils ; one of which, called 

 the ordinary ray, follows the common laws of refraction, agreeably 

 to the preceding tables, whilst the other, called the extraordinary 

 ray, obeys very different laws. This phjenomenon is produced 

 in all transparent crystals, whose primitive form is neither a cube 

 nor a regular octohedron. The division of the beam is greater or 

 less, according to the nature of the crystal, and the direction in 

 which it is cut. But of all known substances, that which pro- 

 duces this phaenomenon in the most energetic manner, is the 

 rhomboidal carbonate uf lime commonly called island spar. 



2. Of the action of the different coloured rays. If the white 

 •sunbeam, admitted through a small hole of a window-shutter into 

 a darkened room, be made to pass through a triangular prism of 

 glass, it will be divided into a number of splendid colours which 

 may be thrown upon a sheet of paper. Newton ascertained, that 

 if this coloured image, or spectrum as it is called, be divided into 

 360 parts, the red will occupy 45, the orange 27, the yellow 48, 

 the green 60, the blue 60, the indigo 40, and the "iolet 80. The 

 red rays being least bent by the prism, from the direction of the 

 white beam, are said to be least refracted, or the least refrangible ; 

 while the violet rays being always at the other extremitv of the 

 spectrum, are called the most refrangible. According to Dr. 

 Wollaston, when the beam of light is only l-20th of an inch 

 broad, and received by the eye at the distance of 10 feet, through 

 a clear prism of flint-glass, only four colours appear; red, yel- 

 lowish-green, blue, and violet. 



If the differently coloured rays of light thus separated by the 

 prism, be concentred on one spot by a lens, they will reproduce 

 colourless light. Newton ascribes the different colours of bodies, 

 to their power of absorbing all the primitive colours, except the 

 peculiar one which they reflect, and of which colour they there- 

 fore appear to our eye. 



According to Sir William Herschel, the different coloured rays 

 possess very different powers of illumination. The lightest green, 

 or deepest yejlovv, which ure near the centre, throw more light on 

 a printed page than any of the rays towards either side of the 

 spectrum. Sir H. Davy remarks, that as there are wore green 

 rays in a given part of the spectrum than blue rays, the difference 



of 



