CIRCULAR POLARIZATION OF LIGHT. 85 
bited throughout noeffect uponthe rectilinearly polarized light, although, 
in order to increase the difference of heat, I was continually cooling its 
upper end with sulphuric ether, 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+ lines, and that of 
the other 134 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 phenomenon 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 phenomenon 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 arrived from below occupy the lower spot. By heating 
the plate so that its lower part constantly preserves the strongest heat, 
the progress of the phenomena 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 the colours which fluor spar acquires by 
rapid cooling, “ Fluor spar was very slightly affected.” 
