POISONS THEIR EFFECTS AND DETECTION. 
to minute foreign particles and the occlusion of mother liquor and air, is 
still visible. So that the vanishing of the dark lines is the only indica¬ 
tion that can be relied upon. 
(b) Method of Due de Chaulnes.—When a transparent plate or 
crystal, ground so that the upper and lower surfaces are truly plane and 
parallel, or a natural crystal having similar surfaces, is viewed by the 
microscope the focus alters. The degree of alteration or change ( d ) 
is connected with the thickness (£) of the crystal or plate, and with its 
t 
refractive index (/x), expressed mathematically as follows : /x=-- A 
t — a. 
glass plate with parallel surfaces is placed on the stage which has a scratch 
or other defining mark on its upper surface ; this reference mark is 
carefully focussed, and the mm. number on the milled head of the fine 
adjustment noted. The crystal is now laid in position over this mark, 
which will now be found to be out of focus. The objective will have to 
be raised : this second reading number is also recorded. The upper 
surface of the crystal is now focussed. The difference between the 
first and last reading, that is, the two readings, of surfaces in which the 
light does not pass at all through the crystal gives t , the actual thick¬ 
ness of the crystal. The difference between the second and third reading 
gives the apparent thickness of the plate, say t ', and the desired re- 
t 
tractive index is obtained by dividing t by t', i.e. [i= —. 
t 
Of course, with regard to double-refractive crystals, by this method 
only the mean refractive index is obtained. 
It is not necessary to examine the crystals in air; they may be covered 
by thin glass and Canada balsam. The index of the combined layer is 
1-53, and allowance must be made by deducting 0-53 from both t and d ; 
or if any other fluid is used of known refractive index a correction 
on the same principle must be used. 
The immersion method may be conveniently studied, as suggested by 
Chamot, by observing minute droplets of oil in water, and minute bubbles 
of air in mucilage or similar fluid. The Abbe condenser is raised so as 
to be as near as practicable to the lower part of the slide, and the drop¬ 
let or bubble sharply focussed ; the iris diaphragm is closed sufficiently 
to make the contours visible. In both bubble of air and disc of oil 
there will be a bright centre surrounded by a more or less dark ring. 
In the two cases in question we have : 
(a) Oil, higher refraction than the water. 
( b) Air, lower refraction than the mucilage. 
On removing the condenser and swinging the mirror to one side : 
(a) Higher refraction —the bright disc moves in the same direction as 
the mirror. 
