198 Royal Society : — 



-JL_th of a grain only, and haAdng produced ten crystals of nearly 

 equal size, of course each one, possessing distinct and decided optical 

 properties, could not represent inore than the Too-o-ot^ V^^^ ^^ ^ 

 grain ; in fact, it really represents much less, inasmuch as one por- 

 tion of the strychnine is converted by substitution into a soluble 

 hydriodate, and of course remains dissolved in the liquid. 



In order to operate in this experiment, it is merely necessary to 

 use diluted spirit of wine, about in the proportions of one part of spirit 

 to three of water, as the solvent medium, and to employ the smallest 

 possible quantity of the tircture of iodine as the reagent, and after 

 applying heat for a short time, to set in repose. On spontaneous 

 evaporation or cooling, the optical crystals deposit themselves, and 

 may be recognized by the polarizing microscope, according to the 

 description given of this substance in a former notice to the Society 

 in Jmie last. 



I remain, &c., 



W. Bird Herapath. 



"Dynamical Illustrations of the Magnetic and the Heli9oidal 

 Rotatory Effects of Transparent Bodies on Polarized Light." By 

 Prof. W. Thomson, F.R.S. 



The elastic reaction of a homogeneously strained solid has a 

 character essentially devoid of all hehcoidal and of all dipolar asym- 

 metry. Hence the rotation of the plane of polarization of light 

 passing through bodies which either intrinsically possess the heli9oi- 

 dal property (syrup, oil of turpentine, quartz crystals, &c.), or have the 

 magnetic property induced in them, must be due to elastic reactions 

 dependent on the" heterogeneousness of the strain through the space 

 of a wave, or to some heterogeneousness of the luminous motions* 

 dependent on a heterogeneousness of parts of the matter of lineal 

 dimensions not infinitely small in comparison with the wave length. 

 An infinitely homogeneous solid could not possess either of those 

 properties if the stress at any point of it was iafluenced only by 

 parts of the body touching it ; but if the stress at one point is 

 directly influenced by the strain in parts at distances from it finite 

 in comparison with the wave length, the heli9oidal property might 

 exist, and the rotation of the plane of polarization, such as is obsei-ved 

 in many hquids and in quartz crystals, could be explained as a direct 

 dynamical consequence of the statical elastic reaction called into 

 play by such a strain as exists in a wave of polarized light. It may, 

 however, be considered more probable that the matter of transparent 

 bodies is really heterogeneous from one part to another of lineal di- 

 mensions not infinitely small in comparison with a wave length, than 

 that it is infinitely homogeneous and has the property of exerting 

 finite direct "molecular" force at distances comparable with the 

 wave length : and it is certain that any spiral heterogeneousness of a 

 vibrating medium must, if either right-handed or left-handed spirals 



* As would be were there different sets of vibrating particles, or were Rankine's 

 important hypothesis true, that the vibrations of luminiferous particles are directly 

 Selected by pressure of a surrounding medium in virtue of its inertia. 



