Intelligence and Miscellaneous Articles, 79 



indigo fluorescence in the direction perpendicular to it. The three 

 crystallized bodies quartz, spar, and fluorine, viewed with respect 

 to the illumination, represent three types, to which all transparent 

 bodies may be referred. To cite only one example, not yet re- 

 marked, pure naphthaline dissolved in alcohol or rectified essence 

 of petroleum has a quinic fluorescence of a very bright indigo-blue. 

 Spectral analysis of this light gives a very intense blue band ex- 

 tending from the line Gr to H and dominating the other colours of 

 the spectrum, which it also includes. 



I must also mention the curious effects obtained with prisms of 

 chilled glass. The thread of polarized light which passes through 

 them gives a luminous trace, white and partially polarized at cer- 

 tain points, while at others it is neutral and coloured yellowish 

 green or bluish green, according to the fluorescence of the glass 

 used. Without entering into further details, it is seen that these 

 effects depend on the double refraction which the ray of light 

 undergoes, and on the direction of the plane of polarization of the 

 illuminating pencil. 



To complete these observations I will add a few words on the 

 photometric experiments by which I measured the proportion of 

 polarized light contained in the rays emitted by a liquid illuminated 

 by a pencil of natural light. If the liquid were not fluorescent, the 

 polarization would be total when we look normally in any plane 

 passing through the axis of the pencil, if it be admitted, as I have 

 said above, that the trajectory of an aether particle on the visual 

 line is not any thing else but the projection of the circle which is 

 the envelope of all the ellipses with variable orientation which re- 

 present the motion of the aether in a ray of natural light. 



The verification of this law would present no difficulty, if the 

 inevitable fluorescence of the liquid did not add to the illumination 

 a proportion of neutral light — constant, it is true, but of which it is 

 necessary to take account. 



I operated as follows, with a photometer the general arrange- 

 ments of which reproduce those adopted by MM. Bernard and 

 Edm. Becquerel. 1 view the illuminated pencil through a Mcol, 

 of which the principal section is at first normal to it ; and I make 

 its light equal to that received from a lamp into a prism with total 

 reflection after passing through two JNicols — the first movable, the 

 second fixed, and their principal sections coinciding. This done, I 

 extinguish the portion of polarized light emitted by the illuminated 

 liquid by turning the first prism through 90°. In order to restore 

 the equality of the lights, it is then only necessary to turn the 

 movable Nicol through a certain angle, which serves to vary the in- 

 tensity of the light for comparison. 



Let a and a! be the angles of rotation which have restored equa- 

 lity of the images when the pencil was viewed, first normally, and 

 then in a direction making an angle <o with the axis of the illumi- 

 nating pencil. If / denote the proportion of fluorescent light, and 

 m the light totally or partially polarized which comes from the 

 lateral propagation of the luminiferous motion, we shall have the 

 following equalities : — 



