J 859.] as investigated hy Light. 101 



line of the optic axes is the same for every colour, in any given crystal 

 species, but that the angle of the optic axes for each colour may be 

 different. Furthermore, in this system, the axes of optical elasticity, 

 and the mean lines, as well as the planes containing the optic axes, 

 coincide in direction with the morphological axes. 



A beam of polarized light was employed to throw on a screen the 

 stauroscopic phenomena produced in sections of crystals, cut perpen- 

 dicularly to their optic axes, or to the first mean lines of these ; and, 

 by the use of absorbing coloured glasses, the different optic axes were 

 shown to diverge more for some colours than for others in the prismatic 

 system. 



Thus Rochelle salt exhibits the centres of the rings or brushes as far 

 more divergent for the red rays than for blue, while in aragonite the 

 converse is the case, though not in so eminent a degree. Mellate of 

 ammonia (and Brookite also) exhibit (as shown by Grailich) the 

 wonderful fact of a divergence of the red rays with the optic 

 axes in the plane x z, while the optic axes for the blue rays lie in 

 the plane y z, and those for green, converge at the centre, into a 

 uniaxial system. 



The more complicated phenomena exhibited by sections of crystals 

 belonging to the clinohedric {oblique and anorthic) systems were next 

 made the subject of illustration, partly by diagrams and in part too by 

 experiment. In the monoclinohedric (the singly leaning prismatic 

 system), there is one plane of morphological symmetry, and it con- 

 tains the two crystallographic axes that are inclined to each other; the 

 third is called "the axis of symmetry," and is the only true crystal- 

 lographic axis in this system, that is fixed by morphological condi- 

 tions. The laws of the (WsirihwiAon^and of the dispersion of the optic 

 axes in this system, as given by Angstrom [Beer's Ilohere Optik, 

 and Grailich's Translation of Professor Miller's Crystallography], 

 are — lst,That this morphological axis is also one axis of polarisation* for 

 all colours, without dispersion {i.e., without any divergence of the 

 directions of vibration of the rays for different colours) ; but that 

 the other two axes of polarisation are rectangular axes, and are 

 dispersed for^ different colours differently in the plane of symmetry ; 

 and 2ndly [Angstrom], there are three different cases peculiar to this 

 system possible : gypsum illustrates one of these, that namely, in which 

 the morphological axis is the second mean line, and the axes of 

 the blue and red rays stand to each other in the positions of 

 B R B R. 



Borax illustrated another of these cases, where the planes of the optic 

 axes for the blue and for the red were so dispersed in the plane of 



■p -r> 



symmetry as to be crossed thus : ^ t> the morphological axis being 



the^^r^^ mean line. 



Finally, Adularia illustrates the position of the optic axes, in which 



♦ The use of the word axu of polarisation instead of axis of elasticity, in this 

 case is afterwards explained. 



