134 



CEYSTALS CUT ACROSS THE AXIS 



laminte whose thicknesses are the differences of those employed in the experi- 

 ment. This supposes that the hamina; are of the same kind. If they are not, 

 the actual thicknesses arc not to be employed in the calculations, but what may 

 be called the hi-refr'mgent equivalents of thickness — that is to say, their measured 

 thicknesses divided by the numbers expressing their chromatic relation to the 

 plates of air which give Newton's rings — which latter numbers may be called 

 their chromatic equivalents. If, then, the difference of these quotients, multi- 

 plied by th-G chromatic equivalent corresponding to the greater quotient, is 

 ^vithin the limits at which the crystal to which the greater quotient belongs 

 gives colors, the combination will give the color belonging to the value of that 

 product. 



If the lamina? belong to crystals of which one is positive and the other 

 negative, they are not to be crossed in this experiment, but their principal sec- 

 tions must be parallel. This furnishes an easy test for determining whether a 

 given crystal 'is positive or negative. Having prepared a lamina of the crystal 

 to be examined, (which may be of any convenient thickness,) apply it upon 

 lamina? of Iceland spar of different thicknesses, with the principal sections suc- 

 cessively parallel and crossed. If the colors appear v>^hen the planes are parallel, 

 the signs are opposite, since, either plate being too thick to produce color alone, 

 the sum of their effects cannot, of course, do so. If the crystals are of similar 

 sign, the colors will appear when the planes are crossed. 



Another class of chromatic effects produced by crystalline plates viewed in 

 polarized light was first observed by Dr. WoUaston in Iceland spar, in vrhich 

 the display is, perhaps, the most brilliant. In these cases, the crystal is cut 

 perpendicularly across the axis. The arrangements for observation are the same 

 as in the experiments already described. If a mirror be employed as an 

 analyzer, and be turned to azimuth 90° before the introduction of the crystalline 

 plate, no light will, of course, be reflected to the eye. But the moment the 

 crystal is introduced a system of concentric rings will make its appearance, 

 colored with the richest conceivable tints, and marked by a black cross, whose 

 ;irms are in the plane of reflection, and at right angles to it, passing through the 

 centre. 



/^. 



Fig. 16. Fig. 17. 



The ends of these arms are enlarged, and have the appearance of brushes. 

 If the analyzer is transparent, another set of rings may be seen by the trans- 

 mitted light, in which the colors will be complementary to the former, and the 

 cross wiM be white. As the analyzing mirror is revolved in azimuth, the colors 

 fade and a new set of rings gradually appears with colors complementary to the 

 tirst, and distinguished by a white cross. In short, in this case, the colors before 

 transmitted are reflected, and those before reflected are transmitted. The annexed 

 ligures exhibit the two aspects of the rings which have been just described. 



