LAWS OF DOUBLE EEFRACTIOX. 69 



lion, arising from the unequal dispersion of the intermediate rays (the 

 green, <tc.). These outstanding colors, as they were termed by Profes- 

 sor Robison, form the residual, or secondary spec-trim. 



Dr. Blair, by very ingenious devices, succeeded in producing an 

 object-glass, corrected by a fluid lens, in which this aberration of 

 color was completely corrected, and which performed wonderfully 

 well. 



The dispersion produced by a prism may be corrected by another 

 prism of the same substance and of a different angle. In this case 

 also there is an irrationality in the colored spaces, which prevents the 

 correction of color from being complete ; and hence, a new residuary 

 spectrum, which has been called the tertiary spectrum, by Sir David 

 Brewster, who first noticed it. 



I have omitted, in the notice of discoveries respecting the spectrum, 

 many remarkable trains of experimental research, and especially the 

 investigations respecting the power of various media to absorb the light 

 of different parts of the spectrum, prosecuted by Sir David Brewster 

 with extraordinary skill and sagacity. The observations are referred 

 to in chapter iii. Sir John Herschel, Prof. Miller, Mr. Daniel, Dr. 

 Faraday, and Mr. Talbot, have also contributed to this part of our 

 knowledge.] 



CHAPTER Y. 



DISCOVERY OF THE LAWS OF DOUBLE REFRACTIOX. 



E laws of refraction which we have hitherto described, were sim- 

 -*- pie and uniform, and had a symmetrical reference to the surface 

 of the refracting medium. It appeared strange to men, when their 

 attention was drawn to a class of phenomena in which this symmetry 

 was wanting, and in which a refraction took place which was not even 

 in the plane of incidence. The subject was not unworthy the notice 

 and admiration it attracted ; for the prosecution of it ended in the dis- 

 covery of the general laws of light. The phenomena of which I now 

 speak, are those exhibited by various kinds of crystalline bodies ; but 

 observed for a long time in one kind only, namely, the rhombohedral 

 calc-spar ; or, as it was usually termed, from the country which sup- 

 plied the largest and clearest crystals, Icdand spar. These rhombo- 



