depolarisation of light, &c\ 45 
4. When there is an approach to a neutral axis, as in gold- 
beaters skin, &c. 
5. When the crystal depolarises, or restores only a part of 
the polarised image, as in a film of sea-weed and a film from 
the partan. 
6. When the crystal depolarises luminous sectors of nebu- 
lous light, as the oil of mace. 
7. When the crystal restores the vanished image, but 
allows it to vanish again during the revolution of the calca- 
reous spar. 
1. The first of these modes of depolarising light admits of 
an easy and satisfactory explanation. 
Let Rr, fig. 3, be the light of a candle completely polarised 
by reflection in the direction rS from the surface of the trans- 
parent body AB. If this light is viewed through a prism of 
calcareous spar CD,* when its principal section is neither 
coincident with, nor perpendicular to the plane of reflection 
RrS, two images of the candle will be seen ; but upon turning 
round the prism CD, one of the two images will vanish alter- 
nately in every quadrant of the circular motion of the prism. 
Let the prism therefore be fixed in the position which it has 
when one of the images has vanished, in which case the prin- 
cipal section will be either parallel or perpendicular to the 
plane RrS. If a rhomb of calcareous spar MNOP is now in- 
terposed, so that the principal section MN is either parallel 
or perpendicular to the plane RrS, the vanished image will 
still be invisible. Upon turning round the rhomb MNOP, the 
* Another prism is represented in the figure for the purpose of correcting, as much 
as possible, the refraction and dispersion of the prism of calcareous spar. 
