836 SUMMARY OF CURRENT RESEARCHES, ETC. 
The trough consists of a metallic vessel provided with a thin exten- 
sion at one end near the bottom furnished with glass-covered apertures, 
above and below. The body of the fish between the gills and tail is 
wrapped with a strip of soft cloth, and the trough being filled with 
water, the fish is placed therein, with its tail projecting into the extension 
between the glass covers. The tank is arranged on the microscopic stage 
with the tail of the fish in position for examination. So long as the fish 
remains quiescent all goes well, and the beautiful phenomenon may be 
witnessed with great satisfaction ; but the subject soon becomes im- 
patient, and at the most inopportune moment either withdraws its tail 
from the field or jumps out of the tank, thus causing a delay which is 
sometimes embarrassing. 
The uneasiness of the fish is caused partly by its unnatural position, 
and partly by the vitiation of the water. The latter trouble has been 
remedied by the writer by inserting a discharge-spout in one end of the 
trough, and providing a tube for continually supplying fresh water. 
The other difficulty has been surmounted by providing two wire grids, 
each having spring clips at their ends for clamping the wall of 
the tank. These grids are pushed downward near the body and head 
of the fish, so as to closely confine the little prisoner without doing it 
the least injury. With these two improvements the examination may 
be carried on comfortably for an hour or more. 
In fig. 92 is shown a simple device for dark-ground illumination. 
Although it does not take the place of the parabolic illuminator or the 
spot-lens for objectives of low angle, it answers an excellent purpose. 
To a metallic slide A, having a 
central aperture surrounded by 
a collar, is fitted a funnel B, of 
bright tin or nickel-plated metal, 
which is provided with a down- 
wardly projecting axially arranged 
wire, upon which is placed a 
wooden button capable of sliding 
up or down the wire, the button 
being of sufficient size to pre- 
vent the passage of direct light to the objective. The light by which 
the illumination is effected passes the button, and, striking the walls of 
the conical reflector, is thrown on the object.” 
Artificial Sea-water.* — Dr. D. Levi-Morenos has some notes on the 
composition of artificially prepared salt water as used with success in 
aquaria, and in keeping oysters, for instance, in good health. Gosse’s 
recipe suggested the following proportions : — Sodium chloride 100, mag- 
nesium sulphate 8*8, magnesium chloride 14 ’3, potassium chloride 3. 
These salts, dissolved and filtered, were added to fresh water till the 
average density of natural salt water was reached. In Perrier’s aqua- 
rium the water contained the following salts in the proportions stated : — 
Sodium chloride 78, magnesium sulphate 5, magnesium chloride 11, 
potassium chloride 3, calcium sulphate 3. 
* Neptunia, i. (1891) pp. 162-4. 
Fig. 92. 
