transmitted through crystallized Bodies, 215 
results already mentioned, gives —-§ for the thickness of a 
plate of topaz that would produce a fringe of the same mag- 
nitude. Hence the thicknesses of ice and topaz that give rings 
of equal size are as to or as 8.95 to 1, which is nearly 
the inverse ratio of [m — 1)^ in ice to (w' — 1)^ in topaz, m 
and m' being the indices of refraction. If we take tn = 1.307 
and m'= 1.6^6, this ratio will be nearly as 8.9 to 1. In these 
experiments the two oval central spots were distinctly seen. 
Light transmitted at an angle of through a plate of ice 
1.25 inches thick gives rings of the same size as when it is 
transmitted at an angle of 60° 38' through a plate of topaz 
of an inch thick. By calculating the real thicknesses 
in the direction of the transmitted light, it will be found that 
the thicknesses at which ice and topaz produce rings of the 
same magnitude are as 8.4 to 1, a ratio not very remote from 
that of (m — 1 ) ^ in ice to (w' — 1 in topaz. 
Light transmitted at an incidence of 36° through a plate of 
ice of an inch thick gave rings twice as large as those 
shewn by a plate of topaz thick. These thick- 
nesses will be found, after reduction, to be as 8.2 to 1, a ratio 
more remote than any of the former from that of [?n — 1 
to — 1)^. 
A plate of ice taken from the surface of a pool of water did 
not appear to depolarise light, when it was incident perpendi- 
cularly : but when the angle of incidence was considerable, 
the light was depolarised in every direction, and the coloured 
rings appeared even at great obliquities. 
9. Sulphate of potash, A plate of sulphate of potash tVo^o 
of an inch thick gave fringes of colour, each of which was 4° 
in breadth, while another plate tqVo thick gave 
