116 University of California Publications in Zoology (Vou. 
a relay or similar apparatus for carrying the heavy current. 
Again, in filling the tube with glycerine care should be taken to 
prevent glycerine from entering the part of the bend where the 
cork float rests on the mereury, for it is exceedingly difficult 
thoroughly to dry the mereury if once wet. All rubber must be 
avoided in order to prevent tarnishing of metal and consequently 
poor contacts. With proper care and experiment it is possible 
to keep the temperature constant to within about one-tenth of 
a degree centigrade. 
Tn a large box without artificial circulation the temperature 
in all parts is not exactly the same, though very nearly constant. 
The microscope is usually a degree or so colder, apparently 
because of radiation from the exposed part. Accordingly the 
upper exposed end should be insulated with a good non-conductor, 
as shown in the photographs. The covering was removed from 
the binocular in order to make the figure clearer. 
Since a camera lucida cannot be used in the ordinary way, 
it was found practicable, and most convenient, to remove the 
mirror from the camera and to use instead a large mirror sup- 
ported over a small shelf at one end of the box (pls. 16 and 17). 
Although the magnification is greater, because of the increased 
distance of projection, the method is still satisfactory. 
CIRCULATION SLIDE 
The circulation slide of glass, perhaps the most important 
piece of apparatus (now being made by the Spencer Lens Co., 
of Buffalo, N. Y.), is shown in detail in figures I to L and can 
also be seen in plates 16 and 17. The unassembled parts, placed 
however in their relative positions, are represented in figure I, 
the slide put together is seen in longitudinal section in figure J, 
and as set up on the stage in figure L. 
The complete slide consists of several parts, shown in figure I. 
(1) A polished lower slide (Ir. sl.) of glass, one inch by three, 
and about 4, inch thick, is perforated by two bolt-holes (bolt 
hl.), which on the under side are countersunk to receive the heads 
of two short bolts. (2) A second polished upper slide (wp. sl.), 
of the same dimensions as the lower and with corresponding bolt- 
holes is the most important part of the apparatus, being the one 
