422 HISTORY OF OPTICS. 



topaz, under the circumstances now described, ex- 

 hibited themselves in the form of elliptical rings, 

 crossed by a black bar, "the most brilliant class of 

 phenomena," as he justly says, " in the whole range 

 of optics." In 1814, also, Wollaston observed the 

 circular rings with a black cross, produced by 

 similar means in calc-spar ; and M. Biot, in 1815, 

 made the same observation. The rings, in several 

 of these cases, were carefully measured by M. Biot 

 and Sir D. Brewster, and a great mass of similar 

 phenomena was discovered. These were added to 

 by various persons, as M. Seebeck, and Sir John 

 Herschel. 



Sir D. Brewster, in 1818, discovered a general 

 relation between the crystalline form and the optical 

 properties, which gave an incalculable impulse and 

 a new clearness to these researches. He found that 

 there was a correspondence between the degree of 

 symmetry of the optical phenomena and the crystal- 

 line form; those crystals which are uniaxal in the 

 crystallographical sense, are also uniaxal in their 

 optical properties, and give circular rings; those 

 which are of other forms are, generally speaking, 

 biaxal; they give oval and knotted isochromatic 

 lines, with two poles. He also discovered a rule for 

 the tint at each point in such cases; and thus 

 explained, so far as an empirical law of phenomena 

 went, the curious and various forms of the coloured 

 curves. This law, when simplified by M. Biot\ made 



3 Mem, Inst. 1818, . 192. 



