bited throughout no effect upon the reclilinearly polarized light, although, in order to increase the difference of heat, I was continually cooling its upper end with sulphuric æther, whilst the lower end stood upon the hot steel plate[1]. 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 longitudinal vibrations of long strips of glass. Although in the experiments made with reference to this, the cross of the Iceland spar figure appeared 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 11½ lines, and that of the other 13¼ 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 phænomenon 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 phænomenon 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 phænomena must of course be more simple.
The action of a determined point of a cooled or compressed glass as 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.
