POLARISATION OF LIGHT. 



59 



Fig. 72. 



distances from it, cannot fail to strike 

 the observer with surprise and admira- 

 tion. 



" The tints exhibited by each crystal 

 van% of course, according to its thick- 

 ness, but the range of tint in the same 

 plate, and at the "same thickness, gene- 

 rally amounts in the largest crystals to 

 three of the orders of colours in New- 

 ton's scale. The central portion, and the 

 two squares above and below it, have in 

 general the same intensity, while the four 

 segments round the central portion, and 

 some of the parts beyond each of the 

 squares, are also isochromatic. In 

 the central part the colours have a de- 

 cided termination ; but towards the sum- 

 mit of the prism their outline is less 

 regular, and less distinctly marked ; 

 though this irregularity has also its 

 counterpart at the other termination. 

 A part of these irregularities is some- 

 times owing to the longitudinal striae on 

 the natural faces of the crystal, so that by 

 carefully grinding these off, the beauty 

 and regularity of the figure is greatly 

 improved. 



" In order to ascertain the order of the 

 colours polarised by the crystal, and ob- 

 serve in what manner they passed into 

 one another, I transmitted the polarised 

 light in a direction parallel to one of the 

 diagonals of the quadrangular prism, 

 and thus obtained, as it were, a section 

 of the different orders of colours, from 

 the zero of their scale. The result of 

 this experiment, which is shown infg. 73, 



larising powers. The difference in the 

 polarising powers is well shown by the 

 variation' of tint; and the difference of 

 refractive power may be obsened with 

 equal distinctness by examining the 

 crystal with the microscopy under fa- 

 vourable circumstances of illumination, 

 when the outlines of the symmetrical 

 forms shown in Jig. 72 will be clearly 

 visible. 



" In examining the splendid arrange- 

 ments of tints exhibited in the figure, 

 the perfect symmetry which appears in 

 all its parts is particularly remark- 

 able. The existence of the curvilineal 

 solid in the centre ; the gradual di- 

 minution in the length of the circum- 

 scribing plates, in consequence of which 

 they taper, as it were, from the angles 

 of the central rectangle to the truncated 

 angles at the summits ; but, above all, 

 the reproduction of similar tints on each 

 side of the central figure, and at equal 



