60 M. J.-L. Soret on Polarization 



specularly by the glass and by the surface of the liquid is inter- 

 cepted, and scarcely any light passes but that diffused by the 

 globules of the blood ; with the spectroscope the two black bands 

 characteristic of the spectrum of oxygenated blood are seen most 

 distinctly. The experiment can also be made with filter-paper 

 wetted with blood — but is then less conclusive, because the white 

 fibres of the paper contribute largely to the diffusion. 



In quitting this subject, let us observe that, to the explana- 

 tion of the colour of bodies by fluorescence, objections can be 

 made which it seems difficult to answer. A plate of obsidian 

 glass roughed gives by diffusion a residue of non-polarized 

 light ; if it is due to fluorescence, why is it not seen when the 

 obsidian is polished, in the same way as with a plate of uranium 

 glass polished the light emitted by fluorescence is distinguished 

 perfectly ? and how is it to be explained that a transparent body 

 (such as quartz) which manifests no fluorescence in its interior, 

 or at its polished surface, can take isochromatic fluorescence 

 when reduced to powder, or simply when its surface is roughed ? 



A Study of some Crystallized Substances. 



I return now to the question whether a medium perfectly 

 homogeneous and not fluorescent can be illuminated by the 

 passage of a pencil of rays. This point appears to me im- 

 portant, because lateral propagation of light in this case is 

 contrary to what has been generally admitted in the theory 

 of undulations. 



Crystallization is a very effective means of purifying sub- 

 stances; and good crystals present a high degree of homoge- 

 neity. Still this is by no means absolute ; and it is well known 

 that, except in very rare cases, the passage of a pencil of sun- 

 light reveals defects (flaws or the presence of foreign corpuscles) 

 in the interior of crystals that appear perfectly limpid in diffused 

 light. The following are the results I have obtained in study- 

 ing illumination in some crystallized substances : — 



Quartz. — Of this I studied a great number of specimens*. 



They exhibit the most diverse properties with respect to illu- 

 mination ; I will mention a few examples of this. 



1. Most frequently fine specimens of hyaline quartz, suffi- 

 ciently pure to be employed in the construction of prisms or 

 other optical apparatus, are not absolutely homogeneous ; and 

 when traversed by a pencil of sunlight, phenomena of illumina- 



* A portion of these belong to the Geneva Museum ; but the most re- 

 markable came from a collection of quartz objects acquired at the Vienna 

 Exhibition by M. Rymtowt-Prince, who kindly permitted me to study 

 them. 



