201 
transmitted through aystallized Bodies, 
Plates of mica, however, while they possess the properties 
of all depolarising crystals, exhibit phenomena peculiar to 
themselves. If the neutral axis GH of a plate of mica is 
inclined forwards so as to make a considerable angle with 
the horizon, the image that was formerly invisible will 
start into existence, and therefore the neutral axis GH is 
accompanied with an oblique depolarising axis Nw. This 
oblique axis is also possessed by topaz, rock crystal, and 
many other crystallized bodies. In making the same ex- 
periment with the depolarising axis of the mica, I observed 
the image to vanish in the direction Mw and Vp, which I con- 
sidered as oblique neutral axes, but I have since found that 
this was owing to the polarisation of the pencil by oblique 
transmission, a property of light which I had not then disco- 
vered. 
We have hitherto considered the depolarisation of light as 
effected by two separate bodies, one of which polarises the 
incident rays, while the other depriv^es them of the polarity 
which they have thus acquired ; but in all bodies that possess 
oblique depolarising axes, light may be polarised and depo- 
larised by the same crystal. Thus if ABah, Plate V., fig. 7, 
be a plate of topaz having DE for its oblique depolarising axis, 
and if a ray RR' of common light is incident at R' with such 
an obliquity that it is polarised by being reflected at C from 
the posterior surface ah, then the ray rr' will be depolarised in 
its passage from C to r along the oblique axis of depolarisa- 
tion, and the emergent ray rr' will be depolarised light. Hence 
it follows that the angle DC6, which the oblique depolarising 
axis makes with the posterior surface ab, is nearly equal to the 
MDCCCXIV. D d 
