as exhibited in its propagation along plates of glass. 75 
equal plates of glass placed upon the faces of octagonal bars 
of hot iron, the black spaces and fringes will have likewise 
an octagonal form, abstracting the effects which take place 
at the extremities of the plates. Now if light polarised in a 
plane inclined 45 0 to the horizon, is transmitted through this 
system of plates, the fringes will be distinctly seen in the 
four plates A, C, E, G, because their depolarising axes are 
all coincident with the plane of primitive polarisation, but no 
fringes will be seen in the plates B, D,F, H, as their depo- 
larising axes are inclined 45 0 to the plane of polarisation. 
If this system of plates be now turned round the centre O, 
each of them will exhibit its fringes when it comes into the 
positions A, C, E, G. These fringes will gradually disappear 
during the motion of the plates into the positions B, D, F, H, 
where they will cease to be visible. 
Let us now suppose, that the hot iron is applied to the 
centre of a circular plate of glass ABCDEFGH Fig. 19. 
(PI. 111 .) the black spaces will obviously have a circular form 
abed efg h and ABCDEFGH; and as the neutral axis 
of each elementary plate, into which we may suppose the 
circle of glass to be divided, is directed to the axis O, the 
dark positions will still be B, D, F, H, and consequently there 
will be a black cross B bf F, D d h H, having its arms in- 
clined 45 0 to the horizon. This cross will continue in the 
same position during the rotation of the plate about its centre 
O, every elementary plate losing its depolarising power when 
it comes into the lines B bf F, D d h H. 
L 2 
