466 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
persion of solid and liquid bodies which are closely related in optical 
characters. (Use of the apparatus as differential-refractometer.) 
The auxiliary arrangements consist (fig. 78) of — 
(1) A new illuminating apparatus, by which the use of sodium light 
and the light of Geissler tubes (H-light), as well as a quick interchange 
of the two kinds of light, is possible. The 
Fig. 79. illumination with sodium light is effected with 
the help of the reflecting prism N, that with 
H-light by means of the Geissler-tube Q and 
the condenser P, which can be adjusted in height 
by c. A micrometer arrangement for the de- 
termination of dispersion consists of the axial 
clamp H, and the measuring-screw G, with 
divided drum. 
(2) A new heating arrangement, which allows 
of the accurate investigation of liquids up to 
100° C. 
The heating is effected either by a stream 
of hot water at a constant temperature, which 
traverses the apparatus as shown by the arrows 
(fig 78), or by the vapour of boiling water or 
other liquids. The prism shares in the heating. 
It is enclosed on three sides by a hollow casing L, 
through which the stream of water flows first 
before it reaches the upper part for the heating 
of the liquid. 
The warming of the liquid takes place in 
the interior of the glass tube. For this purpose 
there is a silver vessel S, which is attached to 
tiie column M, and can be lowered b} r rack and 
pinion into the liquid and adjusted at any height. 
The course of the hot water through the 
vessel S' is seen in fig. 79. The base-plate of 
S can be approached to within a fraction of a 
millimetre of the face of the prism without 
injuriously affecting the observation of the limiting line, so that the 
temperature indicated by the thermometer represents very rigidly the 
temperature of the liquid. To prevent loss of heat 
Fig. 80. by radiation the liquid is surrounded with a wooden 
case (W in fig. 78), with cylindrical boring and 
opening for the entrance of the light. 
(3) A new vessel for liquids, as suggested by 
a Prof. Ostwald, by which the simultaneous investiga- 
tion of two liquids and the direct determination of 
the difference of refraction and dispersion are 
possible. The glass tube is divided into two com- 
partments by a piece of black glass parallel to the 
divided circle. One compartment contains the normal liquid, and the 
other the liquid to be compared with it. 
With respect to the ordinary use of the apparatus for the determina- 
tion of n D , to the new instrument two accessories have been added, viz. — 
