62 M. J.-L. Soret on Polarization 



pletely polarized, and moreover not equal in every pari of the 

 crystal. Examination with the microscope reveals many little 

 flaws in certain parts of the trace ; but I did not succeed in re- 

 solving the whole of it*. 



I found in another seal of false topaz, from Brazil, the same 

 properties, which are probably characteristic of this variety of 

 quartz. 



4. In smoky quartz, when it is not too dark, the trace is in 

 general observed, but in very various degrees. When it is feeble, 

 polarization is complete. 



In a very fine brown specimen from Siberia, the defects of 

 homogeneity manifest themselves, through transparence to dif- 

 fused light, under the form of cloudy strata, indicating unequal 

 distribution of the colouring-matter. With sunlight the trace 

 is very marked, very incompletely polarized, and with a lens a 

 multitude of defects and particles are distinguished. This in- 

 complete polarization is a character always recognized when 

 we operate on a liquid holding in suspension particles too 

 numerous or of too great a volume. 



5. A small specimen of amethyst cut in the shape of a prism 

 with three sides, and with faces perpendicular to the edges, gave 

 no visible trace in the greater part of the crystal ; but in a few 

 points small flaws are observed. 



The study of these phenomena in quartz is often complicated 

 by its property of chromatic polarization. Thus, by making the 

 pencil of light pass, at a little distance from a face, parallel to 

 the axis, and observing the trace with an analyzer, phenomena 

 of coloration or depolarization can be seen, according to the 

 orientation. 



Rotatory polarization plays also an important part when the 

 pencil is directed along the axis of the crystal. Here is a re- 

 markable instance : — The second seal of false topaz above men- 

 tioned is cut so that its axis coincides with that of the crystal ; 

 it presents at its two extremities faces perpendicular to this axis. 

 When a pencil of polarized light enters by one of these two 

 faces to go out at the other, as the substance has a great power 

 of illumination there is reproduced on a small scale the beautiful 

 experiment realized by M. Lallemand when he employed liquids 

 possessing rotatory power. The phenomenon shows itself in 

 coloured fringes which are displaced in the interior of the crystal 

 when the Nieol is rotated that polarizes the incident light. The 



* In truth, observation with the microscope is somewhat difficult : the 

 luminous pencil, and consequently the trace, has always too large a section 

 for the whole thickness to be brought to the point ; the diffused light 

 with which the entire field of the microscope is washed is opposed to the 

 visibility of very small particles or defects. 



