by Diffusion of Light. 63 



first fringes, on the side where the pencil enters, are very bright 

 and distinct; they grow confused and are effaced towards the 

 other extremity. 



Calcareous Spar. — I have found, like M. Lallemand, that fine 

 specimens of Iceland spar present no power of illumination 

 properly so called. The trace of a pencil of sunlight manifests 

 itself solely by the slight red fluorescence of this substance; 

 and no polarization is observed. But if we operate on certain 

 crystals which exhibit minute defects, we can also obtain a white 

 trace. The double refraction of the spar complicates the obser- 

 vation of the polarization-phenomena; yet, with some attention, 

 it has appeared to me that the facts are in this case conformable 

 to what could have been foreseen. 



Rock-salt. — The only good specimen of rock-salt I have had 

 at my disposal is a very fine prism belonging to the Physical 

 Cabinet of the University of Geneva. In it a pencil of light 

 produces a very slight trace, resolvable with the lens into small 

 bubbles or cavities. The polarization is complete; and the 

 defects are visible or disappear according to the orientation of 

 the analyzer. This is the same as I have already stated for 

 quartz. 



In the other, less pure specimens which I have examined, the 

 trace is visible, but unequal, and the polarization normal. 



Diamond. — I had in possession for a few days some large 

 diamonds which form part of the collection of the Duke of 

 Brunswick, and which had been obligingly lent me by the 

 municipality of Geneva. Unfortunately the season was very 

 unfavourable, and I was only able to avail myself, for the ob- 

 servations, of a few moments during which the sun shone. 

 Besides, the diamonds being cut as brilliants renders the obser- 

 vation very difficult : it gives rise to much false light ; and on 

 account of the reflection of the facets, one may readily confound 

 the luminous trace with one of its images. 



(1) A white diamond of 39 carats gave rise to a trace very 

 visible and nearly white. The greater part of the light emitted 

 by it is due to fluorescence; but I feel certain that another 

 part is due to illumination properly so called ; indeed the trace 

 is partially polarized. 



(2) Some yellow diamonds of 29, 31, 42, and 80 \ carats gave 

 rise to a very visible bluish trace, principally due to fluorescence, 

 but also presenting partial polarization*. 



* To observe the polarization-phenomena, I operated in the following 

 manner. A pencil of sunlight, concentrated by a quartz lens of long focus, 

 entered the dark room ; it was polarized in passing through a Nicol, and 

 then fell upon a screen pierced with a small aperture. A slender pencil 



