CH. xin. REFRACTION EXPLAINED. 109 



at once what is the index of refraction for any particular 

 substance. 



It was this law of the proportion between the two hori- 

 zontal lines in the air and in the denser substance which 

 Snellius discovered. It is expressed in mathematical lan- 

 guage, thus : ' The ratio between the sines of the incident and 

 refracted rays is always the same for the same substance ; ' 

 sine being a mathematical term for the measurement we have 

 been making, which you will understand more fully when 

 you have studied trigonometry. 



Chief Works consulted. Herschel s ' Study of Natural Philosophy ;' 

 Lewes' s 'Biographical History of Philosophy;' Cuvier, 'Hist, des 

 Sciences Naturelles ; ' Bacon, ' Ncvum Organum ; ' Huxley on ' Des- 

 cartes,' Macmillan's Magazine ; ' Encyclopaedia Metropolitana,' art. 

 'Light ;' Herschel's 'Familiar Lectures,' art. 'Light.' 



