53 
depolarisation of light , &c. 
and deflexion, yet we are not entitled to employ this conjec- 
ture in the explanation of phenomena. 
7. In all the preceding cases of depolarisation, the depola- 
rised image continues visible in every part of the circular 
motion of the prism of calcareous spar, but there are cases 
where the vanished image is restored, and again vanishes 
during the revolution of the prism. 
This phenomenon takes place when the polarised pencil is 
depolarised by transmitting it along the short diagonal of a 
rhomb of calcareous spar, or along the axis of a hexaedral 
prism of nitre, or through a parcel of glass plates, or through 
plates of agate and carbonate of barytes, that give a bright 
and a nebulous image. In all these cases only one bright 
image is produced, so that the images must vanish alternately 
in every quarter of a revolution, the only effect of the depo- 
larising body being to polarise the light in a different plane, 
and thus to shift the vanishing place of the images. 
Hence it follows that every body which possesses this kind 
of depolarisation, forms either a bright and a nebulous image, 
like the agate, or a single image, the light of which is all po- 
larised in the same manner. 
I have the honour to be, &c. 
DAVID BREWSTER. 
Edinburgh, October 22, 1814. 
