OPTICS. 



Table of the Refractive Power of Gases, 

 according to M. DULONG, that of Air 

 being 1.000. 



Atmospheric Air > . . . . 1 .000 



Oxygen 924 



Hydrogen 0.470 



Azote 1.020 



Ch'orine 2.623 



Oxide of Azote 1.710 



Nitrous Gas 1 .030 



Hydrochloric Acid 1.527 



Oxide of Carbon 1.157. 



Carbonic Acid 1.526 



Index of 

 Retraction. 



Cyanogen, 2 . 832 



Olefiant Gas 2.302 



Gas of Marshes 1.504 



Muriatic Ether 3.72 



Hydrocyanic Acid 1.531 



Ammonia 1.309 



Oxi-Chloro-Carbonic Gas 3.936 



Sulphuretted Hydrogen 2 . 1S7 



Sulphureous Acid 2.260 



Sulphuric Ether 5.280 



Carburetted Sulphur 5 . 179 



It will appear from a comparison 

 of the preceding table, with that of spe- 

 cific gravities in HYDROSTATICS, Chap. 

 VI., that in very many cases the refrac- 

 tive power increases with the density of 

 the body. In the case of oily sub- 

 stances, or inflammable bodies, how- 

 ever, such as hydrogen, phosphorus, 

 sulphur, diamond, bees' wax, amber, 

 spirit of turpentine, linseed oil, olive 

 oil, camphor, their refractive powers 

 are from two to seven times greater in 

 respect to their density than those of 

 most other substances. Sir Isaac 

 Newton observed this fact with respect 

 to the last five of these substances, 

 which, he says, * are fat, sulphureous 

 unctuous bodies,' and as he observed 

 the same high refractive power in the 

 diamond, he infers, that it is ' pro- 

 bably an unctuous substance coagu- 

 lated.' This law, however, at one time 

 seemed to be overturned by an ob- 

 servation of Dr. Wollaston, that phos- 

 phorus, one of the most inflammable 

 substances in nature, had a very low 

 retractive power ; but Dr. Brewster, 

 confiding in the truth of the law, exa- 

 mined the refractive power of phospho- 

 rus by forming it into prisms and lenses, 

 and he found it to be nearly as high as 

 diamond, and fully twice that of dia- 

 mond compared with its density; an 

 observation which re-established and 

 extended the truth of the general prin- 

 ciple. 



CHAPTER III. DIOPTRICS continued. 



Refraction of Rays by Prisms and Len- 

 ses Burning Glasses Illuminating 

 Lenses. 



THE substance which is most commonly 

 used for refracting the rays of light, 

 both in optical experiments, and in op- 

 tical instruments, is glass. For these 

 purposes it is shaped into solids of the 

 following form, a section of which is 

 shewn i\\/ig. 3. 



