Recent Progress in Theoretical Physics. 



241 



REPOET ON RECENT PROGRESS IN THEORETICAL 



PHYSICS. 



By J. E. DAVIES, A. M, M. D., 



Professor of Physics in the University of Wisconsin. 



Part II. 



THE MAGNETIC ROTATORY POLARIZATION OF LIGHT. 



It is a well known fact that a raj of plane polarized light, 

 vibrating in any azimuth, will, on passing through a lamina of 

 quartz, have the azimuth of that vibration changed by an amount 

 depending upon the thickness of the lamina, and the wave length 

 of the particular kind of light employed. The direction, right or 

 left, of this rotation of the plane of vibration depends upon the 



Note to Pkeviotjs Paper on " Vortex Motion." 



For the production of large-sized vortex riugs, tlie device shown in Fig. 1 



is used by Prof. Tait. It is an ordinary wooden box, with a large circular 



hole cut out of one end, and the other covered tightly with elastic cloth. It 



...^ ^, can be filled with 



Tig.l 



smoke from a 

 couple of retorts, 

 one containing 

 Ammonia and the 

 other Hydrochlor- 

 ic Acid. This will 

 give copious 

 clouds of chloride 

 of Ammonium, 

 which are driven 



out]in vortex rings, on striking the elastic cloth. It has been objected that 

 the rings thus produced do not behave as Helmholtz' mathematical results 

 imply. It is not to be expected that Ihey should ; for Helmholtz' investiga- 

 tion upon vortex motion expressly assumes that the medium in which the 

 rings are formed is a frictionless fluid, which air is not. The riugs are truly 

 air rings, the accompanying smoke merely serving to make them visible. 

 This is finely shown by sending air rings from a second box against the 

 smoke rings already formed. The invisible air rings are made manifest by 

 the jostling of the smoke rings as they are struck by them. The suddenness 

 of this movement is often very striking. 

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