506 NOTES TO BOOK IX. 



power of various media to absorb the light of different 

 parts of the spectrum, prosecuted by Sir David Brewster 

 with extraordinary skill and sagacity. These observations 

 are referred to, p. 395. Sir John Herschel, Prof. Miller, 

 Mr. Daniel, Dr. Faraday, and Mr. Talbot, have also con- 

 tributed to this part of our knowledge. 



(IA.) p. 399. I have, in this part of my narrative, 

 attempted to correct some inaccuracies which were pointed 

 out in the former edition ; and I have omitted some ex- 

 pressions which related to controversies arising out of these 

 discoveries, and which, it was represented, might give pain 

 to the persons concerned. 



I ought to have stated in this chapter that Malus dis- 

 covered the depolarization of white light in 1811. He 

 found that a pencil of light which, being polarized, refused 

 to be reflected by a surface properly placed, recovered its 

 power of being reflected after being transmitted through 

 certain crystals and other transparent bodies. Malus 

 intended to pursue this subject, when his researches were 

 terminated by his death, Feb. 7, 1812. M. Arago, about 

 the same time, announced his important discovery of the 

 depolarization of colours by crystals. 



I may add, to what is said of M. Biofs discoveries 

 respecting the circular polarizing power of fluids, (p. 389,) 

 that he pursued his researches so as to bring into view 

 some most curious relations among the elements of bodies. 

 It appeared that certain substances, as sugar of canes, had 

 a right-handed effect, and certain other substances, as 

 gum, a left-handed effect ; and that the molecular value of 

 this effect was not altered by dilution. It appeared also 

 that a certain element of the substance of fruits which had 

 been supposed to be gum, and which is changed into sugar 



