Royul Institution. 429 



nomena. He has there shown the vahie of this means of tracing 

 changes in chemico-molecular constitution. 



M. Pasteur has carried forward this inquiry into a new channel by 

 tracing a connexion between this property in substances, of circularly 

 polarizing light, and their crystalline character. 



But as it would be impossible to explain the nature of his investi- 

 gations, or their results, without a preUminary knowledge of the 

 meaning of the terms " circular polarization" and " hemihedrism, 

 it was necessary first to enter a little upon the explanation of them. 

 Accordingly a rav was explained as being a direction of hght. 

 having no relations to space which differed from each other in direc- 

 tions perpendicular to its length. Thus without complicating the 

 subject, by using the language of the beautiful wave-theory, a ray 

 might be imagined as a cylinder of minutest diameter, but indefinite 

 length. When such a ray is reflected at a certain angle from glass 

 or such like substance it is split into two; one going into, and 

 through the glass if it be not opake, the other being reflected frona 

 it. These two rays no longer possess the same " absence of sides ' 

 as the original rav. For the one has been as it were flattened down 

 to a " strip," while the other has also been flattened similarly into 

 a " strip," but the latter strip is at right angles in regard to its 

 "flattened plane" to what the other is. A similar bifurcation of 

 the ray is produced in the interior of what are called doubly refract- 

 ing crj-stals. This bifurcation and flattening of the ray is termed 

 '• plane polarization " of it ; and it is so far a true instance of po- 

 larity, as that the two rays have equal and similar properties in 

 opposed directions. 



This was exhibited by the Lime Light. The double image of a 

 small round hole formed by a crystal of Iceland spar was thrown on 

 a screen, and each beam shown to be most capable of reflexion ma 

 plane in which the other was incapable of being reflected at all. The 

 action of the tourmaline as a doubly refracting crystal which absorbs 

 one of the rays was then explained ; and it was shown that the po- 

 sition of the tourmahne in which it intercepted one ray entirely, was 

 exactly the position in which it gave the other ray free passage. 

 The optic axis of a crystal was then defined to be a direction in it 

 along which the light could pass through the crystal without under- 

 going any change whatever. The central ray of a polarized beam 

 of light, traversing a piece of calc-spar along its optic axis, was shown 

 to be intercepted or transmitted by a tourmaline, precisely as if the 

 section of the crystal of calc-spar were away. 



An exception was however stated to exist to this law of the neu- 

 trality of the optic axis. When a section of quartz, cut so that the 

 beam' could career along its optic axis, was put in the path of the 

 polarized ray, it was found that instead of jiermitting the ray to be 

 eclipsed by tiie tourmaline when this was placed in the position to 

 eclipse it, that ray on the contrary fell on the screen endowed with 

 beautiful colour ; and furthermore that the revolution of the tourma- 

 line induced the most brilliant succession of colours, in the order, in 

 the instance exhibited, of red, plum-colour, blue, green, orange, red. 



