CH. xiil. REFRACTION EXPLAINED. 107 



term for the measurement we have been making, which 

 you will understand more fully when you have studied 

 trigonometry. 



Chief Works consulted. HerschePs ' Study of Natural Philosophy ;' 

 Lewes 1 s 'Biographical History of Philosophy;' Cuvier, 'Hist, des 

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

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

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



