47 
depolarisation of light, &c. 
particular position that enables it to double each of these 
pencils; so that four images of the candle l, 2, 3, 4, will now 
be visible. As the rhomb MNOP produces only a very small 
separation of the pencils ST, SV, the two images 1, 2, will 
overlap each other, and resemble only one image at E, while 
the other two images 3, 4, will appear as a single image at F. 
Every thing remaining fixed, let the prism CD be turned 
round in a plane perpendicular to ST. The effect of this will 
be to extinguish one of each of the double images E and F at 
every quarter of a revolution, that is, first the images 1 and 3, 
then the images 2 and 4, then the images 1 and 3 again, and 
last of all the images 2 and 4. Still, however, one image is 
always left at E and another at F, so that when the polarised 
ray rS passes through the depolarising axis MO of the rhomb, 
the two images E and F continue visible in every part of the 
motion of the prism. The depolarisation, therefore, of the 
pencil rS, is nothing more than the polarisation of it in a new 
plane, and the depolarising rhomb MNOP acts in every respect 
like a doubly refracting and polarising crystal. 
2. In the second kind of depolarisation where the human 
hair, or a plate of mica is substituted in place of the rhomb of 
calcareous spar, the phenomena are precisely the same as 
those which have been described in the preceding section, and 
therefore we are necessarily led to suppose that the human hair 
and the mica form two images polarised in an opposite manner, 
like those given by calcareous spar. These two images in- 
deed being produced by the same, or nearly by the same 
refractive power, cannot be rendered visible by any contri- 
vance ; but when we consider that the depolarising axes of 
the mica coincide with the long and short diagonals of its 
