698 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
different liquids, and from these two angular measurements the refrac- 
tive index can be readily found by calculation by means of the well- 
known formula 
e 0 e e 
e m = „ 7-t— • 
e 0 cos' 5 a X e e sim a 
On Work with a Polarization Microscope and a Simple Method 
for the Determination of the Sign of the Double-refraction.* — Prof. 
C. Klein gives a critical and historical account of the various methods 
which have been devised for determining the sign of double-refraction 
both in convergent and parallel light. In treating of the modes of ob- 
servation of the interference figure in convergent light in the Microscope, 
he shows that he was the first to make use of the method in which the 
figure is observed by means of a lens held above the ordinary eye-piece 
of the Microscope. 
Having passed in review the various methods which have been pro- 
posed for determining the sign of double-refraction, such as comparison 
with a plate of known optical character cut at right angles to the optic 
axis, use of Biot’s compensation quartz and gypsum wedges, and of 
retardation plates of gypsum and mica, including the usual quarter-wave 
plate, he comes to the conclusion that the succession of colours obtained 
by the use of a wedge of quartz or gypsum affords the most general, the 
simplest and the most convenient process for determining the character 
of the double-refraction ; because in this succession of colours we have 
the means of replacing, not only for the observation in parallel light, 
all gypsum and mica plates of different tints, but also for observation 
in convergent light, all plates devised in order to give the various and 
distinctive interference phenomena in the different quadrants of the 
interference figure. 
For this purpose the polarization Microscope should be provided 
beneath the upper nicol and above the objective, with a slit running 
from the right in front to the left behind. In the slit can be inserted 
either : — 
(1) A gypsum wedge giving colours of the first order in which the 
smaller axis of elasticity is in the plane of the plate and parallel to the 
edge. 
(2) A similar or quartz wedge giving colours from the first to 
higher orders with similar optic orientation. 
(6) Miscellaneous. 
Progress in Microscopy. — We are glad to read the following note 
in the Journal of the British Dental Association: j - — “ Our Association 
has recently shown its appreciation of the value of microscopical 
research in connection with dental histology by authorizing, at our last 
annual meeting, the formation of a Microscopical Section (as an experi- 
ment for that occasion). We think that the success of that experiment 
is undoubted. The opening address by the president, Mr. Charles 
Tomes, as well as the special discussion and lantern demonstrations 
which followed, were very fully attended, and it is to be hoped that the 
* SB. K. Preuss. Akad. Berlin, xviii. (1893) pp. 221-45. f xiv. (1893) p. 465. 
