217 
transmitted through crystallized Bodies, 
5® 45' : 8° 25' = 
45 
r 000 
61 S 
I 000 
the thickness of topaz that would give the fourth ring a dia- 
meter of 5° 45'. Hence the thicknesses at which* the nitrate 
of potash and topaz produce rings of equal magnitude are as 
to T oooy ^ 3*97 But assuming the rings 
to vary as (m — 1)^, those formed by the nitrate should have 
been larger than those exhibited by the topaz in the ratio of 
636^ : 515^, or nearly 1.88 to 1. Hence the rings formed by 
nitrate of potash are 1.88 x 3*97, or 7.5 times smaller than they 
should be if their conjugate diameters had varied as (w — 1)^.* 
12. Acetate oj lead. This doubly refracting crystal melts at 
a temperature not much greater than that which bees' wax 
requires, and it takes a long time to cool and crystallize. 
When it is formed by heat into a thin film between two plates, 
the crystals shoot from different centres, and exhibit by po- 
larised light the most beautiful alternations of the prismatic 
colours. When the eye is kept at a distance from the plate, 
the colours radiate like the spicula of the salt, and vary at 
every inclination of the plate. 
13. Mother of pearl. The coloured rings are extremely bril- 
liant in this substance when the polarised light is transmitted 
almost perpendicularly ; but they do not appear when it pene- 
trates by an oblique path. 
The other substances, which have already been mentioned 
as affording coloured rings by polarised light, exhibit only 
imperfect segments of the fringes, but in all of them these 
• The thickness of the plates of ice, sulphate of potash, and nitrate of potash, and 
the inclination of the incident pencil were measured in the rudest manner, as my 
object was merely to ascertain in general if there was any connection between the 
magnitude of the coloured rings and the refractive power of the body which produced 
them. 
MDCCCXIV. 
Ff 
