On the Law of Double Refraction. 229 



oddly enough, that though he had two prisms in every other 

 case, he used only one of Iceland spar : he could not there- 

 fore compare the velocities of rays passing in different direc- 

 tions. On comparing his numbers, however, with those of 

 Wollaston and Malus, there is, as Sir David Brewster has 

 observed,* a " surprising discrepancy," so great indeed as to 

 be quite " alarming." After remarking the difficulty of find- 

 ing any explanation of it, Sir David concludes that it must 

 arise from* the different refractive powers possessed by different 

 specimens. But though this cause must operate in some 

 degree, we cannot tell to what extent it is effective, and the 

 discrepancy may notwithstanding be occasioned, in a great 

 measure, by a deviation from the Huyghenian law. The whole 

 question must therefore be reopened, and the ordinary in- 

 dices for the fixed lines of the spectrum must be determined by 

 means of different prisms cut out of the same piece of Iceland 

 spar. 



"Whatever the result may be whether it shall confirm the 

 law of Huyghens, or show that another must be substituted for 

 it it will at least be useful for science, by removing the uncer- 

 tainty in which the subject is at present involved. 



* Phil. Mag., S. 3, VOL. i. p. 8. 

 TaiN. COLL., DUBLIN, Sept. 24, 1842. 



