PRINCIPLES OF CRYSTALLOGRAPHY. 



263 



appoar snperposed ; on account of the dispersion of tlie principal section, 

 both of tlie two images of the axes, and also the arrangement of the 

 colors in both of them, will be unsymmetrical with regard to the princi- 

 pal section, which is marked by a somewhat faint and black beam. The 

 detail of this image of the axes is most simply described by saying that 

 a union of the cases of dispersion, met with in variable intensity in the 

 following system of crystallization, is to be here observed. 



2. MONOCLINIC SYSTEM.— One plane of symmetry. A principal opti- 

 cal section of every color must coincide with the plane of symmetry, so 

 an axis of elasticity of every color must coincide with the axes of the 

 crystal o Y, perpendicular to the plane of symmetry. The two other 

 principal sections, as also the two axes of elasticity lying in the plane 

 of symmetry, are dispersive for the different colors. There are here 

 three possible cases : 



First. The principal section a c, containing the optical axes of a color, 

 coincides with the plane of symmetry, inclined dispersion, {disjyersion 

 inclinee of Descloiseaux.) The general case is, that the analogous principal 

 sections have for all colors very nearly the same position ; in this case 

 the optical axes, for all the colors, lie in the plane of symmetry ; the 

 image of a plate perpendicular to a bisectrix, (convergent light,) on 

 account of the correspondence of the direction of vibration of the plate 

 and. the polarizer, is symmetrical with respect to the black beam joining 

 the image of the axes, (Fig. 30.) 



Secondly. The principal section of the axes is perpendicular to the 



Fi>f.36. 



plane of symmetry; the bisectrix 

 lies in the plane of symmetry, hori- 

 zontal dispersion, {dispersion horizon- 

 tale of Descloiseaux.) In this case 

 c h for positive crystals, and a b for 

 negative crystals, coincide with the 

 plane of symmetry. 



If the general case of the approximate coincidence of similar princi- 

 pal sections for diHerent colors is selected, we see that here the planes 

 of the optical axes are dispersive. The image of tlie axis appears sym- 

 metrical with respect to a beam perpendicular to the line of the optical 

 axes. (Fig. 37.) 



Thirdly. The section of the axes ac and the bisectrix are perpendic- 

 ular to the plane of symmetry ; the prin. /Fy.jr. 

 cipal section a b for positive, and c b for 

 negative crystals, coincide, therefore, 

 with the plane of symmetry ; cross-icise 

 dispersion, {dispersion croisee of Descloi- 

 seaux.) The planes of the axes are dis- 

 persive. 



Under the same supposition as before, " 



the image of the axes will not be symmetrical with regard to any line j 



