CIRCULAR POLARIZATION OF LIGHT. 85 



bited throughout no effect upon the reclilinearly polarized light, altliough, 

 in order to increase the difference of heat, I was continually cooling its 

 upper end with sulphuric aether, whilst the lower end stood upon the 

 hot steel plate*. Sonorous plates vibrating transversely acted neither 

 upon the linear nor the circular incident light. But it is well known 

 that Biot obtained a flash of light between the cross mirrors by the lon- 

 gitudinal vibrations of long strips of glass. Although in the experiments 

 made with reference to this, the cross of the Iceland spar figure ap- 

 peared to me to open, yet those experiments stand in need of being 

 repeated with a better acoustic apparatus. 



8. Difference between the Action of Glass when it is Heating and 

 when it is Cooling. 



Two square plates 3 lines thick, the side of one 11 4- lines, and that of 

 the other 13-^ lines, produced on being heated at first a circular light 

 on the right, and then a rectilinearly polarized one ; on their cooling, 

 however, after they had returned to the rectilinear through the circular 

 one on the right, they produced circular light on the left. The reason 

 of this phaenomenon is as follows : The lower end of the glass plate 

 heated upon the hot steel plate cools when the lamp is taken away 

 quicker than the upper one, to which heat is also communicated by con- 

 duction. After some time therefore the centre of the plate becomes its 

 warmest part. As the lower end, standing upon the rapidly cooled 

 conductor of heat, becomes still cooler, the warmer spot moves upwards 

 until finally the upper angle becomes the warmest. That this is truly 

 the reason of the phsenomenon may be seen by examining the cooling 

 plate between the crossed mirrors. The four white vacant spaces of 

 the diagonals do not disappear on the spot where they had been formed; 

 the lower ones rather move upwards, so that the dark cross becomes 

 changed into two parallels, which are intersected by a perpendicular 

 line. Finally, the central white vacant spaces dislodge the upper ones, 

 whilst those newly aiTived from below occupy the lower spot. By heating 

 the plate so that its lower part constantly preserves the strongest heat, 

 the progress of the phaenomena must of course be more simple. 



The action of a determined point of a cooled or compressed glassas a 

 circularly polarizing apparatus, in the homogeneous rays of the spectrum, 

 gives immediately the elements of determination for the colour which the 

 glass presents in rectilinearly polarized light. 



• Brewster says in reference to tlie colours which fluor spar acquires by 

 rapid cooling, " Fluor spar was very slightly afiected." 



