468 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
with central aperture 0 closed by a glass plate, supports the vessel V, 
52 mra. high and 80 mm. upper diameter, which contains the liquid. 
The section under examination is held by the clamps K on the plate 8, 
whose central part is of glass. Rotation of the section about a vertical 
axis is effected by the screw D, that about a horizontal axis by D'. 
Divisions and verniers N and N' allow these rotations to be read to five 
minutes. 
The author explains the use of the apparatus in determining the 
position of the plane of the optic axes and the character of the double 
refraction in biaxial crystals. 
Regulating d'Arsonval’s Thermostat.* — Dr. M. Melnikow-Raswe- 
dcnkow states that d’ Arson val’s thermostat may be regulated to the tenth 
of a degree by the careful removal or addition of water from the jacket. 
This may be easily effected by inserting into the glass tube, which indi- 
cates the level of the water in the jacket, a siphon tube and a burette 
tube, the former for withdrawing, the latter for adding water. In this 
way the constancy of the temperatare is easily maintained. 
C4) Photomicrography. 
Photographic Technique of Wilson’s Atlas. f — Dr. E. Learning 
gives the following account of the technique used in producing the 
photomicrographs of eggs of Toxojpneustes. The installation used was 
that manufactured by Zeiss of Jena. The adjustment of focus was left 
entirely with Prof. Wilson, as being most familiar with the special 
points desired. The exposure was then made so as to slightly overtime 
the plate, and it was subsequently intensified. Where advisable, Strong’s 
adjustable false stage was used, in order to bring into the same focal 
plane a second or third point of interest, and it was found that, notwith- 
standing the short working distance of a 2 mm. lens, the slide could be 
considerably tilted. The optical combination w r as an Abbe substage 
achromatic condenser 1 N.A., a Zeiss 2 mm. oil-immersion apochromat, 
and projection ocular No. 4. The luminant employed was the electric 
arc, specially modified for the purpose and giving an evenly lighted 
field. As the objects had a light blue tint by transmitted light, iso- 
chromatic plates were used with a coloured screen made by dyeing a 
lantern slide plate, from which the silver salts had been removed with an 
alcoholic solution of tropmolin. 
(5) Microscopical Optics and Manipulation. 
Appearances of Colour on the Boundaries of Colourless Objects 
under the Microscope.^; — Dr. II. Ambronn remarks that it is only when 
the refractive indices of a solid and a liquid are sensibly different from 
each other that the boundary between them is seen under the Microscope 
as a colourless dark line. If, on the other hand, the values for the 
refractive indices only vary by some units in the third decimal place, 
then in white light certain, often very lively, colour appearances are 
seen on the boundary. The author gives the following explanation of 
this phenomenon. 
* Centralbl. f. Bakteriol. u. Parasitenk., l te Abt., xix. (1896) pp. 709-12 (1 fig.). 
+ See Wilson’s Atlas, ante, p. 394. 
% Ber. Verb. d. K. Sachs. Gesell. d. Wiss. Leipzig, i. (1896) pp. 134-40. 
