by Diffusion of Light. (>1 



tion are produced. The trace of the pencil is visible; only it is 

 in general unequal, and more marked in certain places than 

 in others. On examining this trace with a lens or with a mi- 

 croscope, three cases can be distinguished : — 



(a) Very often the trace is due to little defects of crystalliza- 

 tion, to little flaws, which manifest themselves under the ap- 

 pearance of luminous strokes following one or two determinate 

 directions*. Under a vision-angle of 90° the polarization of the 

 light thus diffused is complete, provided the defects of crystalli- 

 zation are very minute ; when the flaws are large enough to be 

 readily visible to the naked eye, the polarization is only partial. 

 If the employment of the lens and of the analyzer be combined, 

 these little defects of homogeneity, very apparent when the 

 analyzer is in the position in which it lets pass the light polarized 

 in the plane of vision, totally cease to be visible when the ana- 

 lyzer is rotated into the extinction-position. 



(b) Sometimes the trace is due to minute bubbles or cavities 

 which are discovered by means of a lens or a microscope. The 

 light is here again completely polarized ; the bubbles become 

 invisible if viewed at a right angle to the pencil, with the ana- 

 lyzer suitably oriented. 



(c) In some specimens, or in some parts of a specimen, I have 

 not succeeded in resolving the trace with a low power of the 

 microscope. But I have no doubt that, in this case again, it 

 owes its origin to imperceptible defects or to foreign particles : 

 indeed no limit can be assigned to their minuteness. 



2. Some specimens of hyaline quartz, very pure and much 

 rarer, are, so to say, destitute of the power of illumination. 



(a) 1 possess a seal of Siberian quartz which is cut in such a 

 way as very well to permit the observations : its general form is 

 that of a six-sided prism with bases perpendicular to the edges. 

 When placed in the dark room and traversed by a pencil of direct 

 sunlight, no trace is to be seen. On concentrating the light by the 

 process described above for the study of the reflecting-power of 

 flames, taking at the same time the greatest precautions to avoid 

 all false light, a slight trace can with difficulty be distinguished. 



(b) Another seal, of Alpine quartz, the shape of which is 

 rather less suitable for the observations, is incontestably illu- 

 minated in certain parts; but in others it has been impossible 

 for me to see the trace. 



3. A very beautiful seal of yellow Siberian quartz (false 

 topaz), appearing quite limpid in diffused light, is brightly illu- 

 minated by a pencil of sunlight ; the trace is bluish, and at first 

 sight might be thought due to fluorescence. But on observing 

 it with some care, the light emitted is recognized to be com- 



* I cited a case of this kind in the Archives, 18/0, tome xxxvii. p. 155. 



