W. P. Riddell on Soleil’s Saccharimeter: 177 
play of colored concentric rings intersected by a rectangular 
black cross, whose arms meet at the centre of the rings. In 
these rings we observe a succession of the seven colors of the 
Spectrum ; and some very singular changes, which need not here 
ing through certain crystals, may usually be traced to the pecu- 
Jar structure or crystallization of said crystal; but quartz, which 
is classed in the same system of crystals with Iceland spar, gives 
us quite a different result when placed in the polariscope. We 
observe a system of prismatic rings, but the black cross in the 
centre is not there, and in its place we have a certain uniform tint 
the hue of which changes successively through the whole range of 
the spectrum, when either the analyzer or polarizer is revolved ; 
for right-handed quartz, if we revolve the analyzer to the right, 
the colors will be seen to succeed each other in the following 
t: red, orange, yellow, green, blue, indigo, violet, red, and so 
on; while for the left-handed quartz they will be seen to succeed 
each other in the contrary order, viz.: red, violet, indigo, blue, 
sreen, yellow, orange, red, and’so on; hence to obtain the same 
' order of succession as in the first case we must revolve the an- 
Kind of quartz is called dertrogyrate, the second levogyrate. 
The width of this cevtral hue, as well as that of the concentric 
the Square root of the thickness of the film of quartz. 
As circular polarization is supposed to result from two sets of 
rectilinear vibrations centrally joined, acting in rectangular planes 
one of the sets being retarded, so colors are su posed to result 
from two sets of rectilinear vibrations, one of which is retarded, 
bat both of which act in the seme plane. 
The application of the foregoing principles for the purposes of 
analysis comes next to be explained. If we take two plates of 
fotemaline, (which by the way must be cut in planes parallel to 
the axis of crystallization,) place them parallel, one with its axis 
Ml vertical, the other with its axis in a horizontal position ; 
permit a ray of common light to strike perpendicular upon 
»—the first will stifle all except the vertical vibrations, while 
the second will stifle all except the horizontal ones; hence no 
light will be permitted to pass through both plates thus placed. 
this position of the two plates of tourmaline, viz.: so as to 
*Xtinguish the light, is termed the azimuth zero, or point of 
Steatest obscuration. If now we place between these two plates 
2 piece of quartz, or any other circularly polarizing substance, the 
effect is immediately noticed from the fact that the light which 
as before obscured by the two tourmaline plates is now seen to 
