178 W. P. Riddell on Soleil’s Saccharimeter. 
pass through the three,—and in order to reéstablish the azimuth 
zero, it is necessary to revolve the analyzer, either to the right or 
left as the case may be, through a certain number of degrees, ac+ 
cording to the thickness of the interposed quartz. If instead of 
the plate of tourmaline, a double refractor be employed fora po- 
larizer, a similar result is observable, except that instead of totally 
shutting out the light at the azimuth zero, we are only able to. 
exclude one image, usually the extraordinary ;i,and so to réestab- 
lish the azimuth zero, we only seek to exclude one of the ima- 
ges, both having been rendered visible by the interposition of the 
quartz; this we do as in the other case, by revolving either the 
analyzer or polarizer. The angle through which it is necessary 
to revolve the analyzer has been found proportionate to the thick- 
ness of the quartz; or if two plates of quartz be interposed, the 
angle of variation will be as the swm of the two thicknesses, if 
. both be right-handed, or both left-handed; or as the difference, 
if one be right and the other left: and the same in regard to all 
muth zero; but it was found no easy matter to judge as accu~ 
rately as was desirable as to this point of greatest obscuration, for 
the analyzer had to be revolved over several degrees ‘before any 
change could be observed. 
A second step was taken by substituting a ray of homogeneous 
light; but still it was found exceedingly difficult to distinguish, 
with a precision sufficiently constant and exact, the precise imut 
of each color; for they seem to flow so readily into each other, 
and besides, of themselves, have something of a margin on the 
spectrum. This defect is very ingeniously surmounted in the 
one left-handed, the other right-handed ; so that any difference 8 
not only doubled, but instead of judging of the identity of any 
certain abstract hue, we have only to identify, by means of the 
compensator, the hues of two juxtaposed semi-discs. 
Soleil's Saccharimeter.—We have now taken a general view of 
the principles on which saccharimetry, by means of circular PO 
larization, depends. It remains briefly to’ speak of the insttt- 
ment constructed by Soleil of Paris, (which I think undoubtedly. 
deserves the first place,) and describe somewhat more minutely 
the process of actual analysis by means of it. aun 
