A DIFFERENTIAL WARM STAGE, 373 
and five inches long. To the same edge is soldered also a 
tongue of copper—in the position shown—an inch long, and 
a quarter of an inch in breadth, A piece of crdinary copper 
fal 
. 1 
bell-wire is now wound round both the copper tongue and 
iron wire for four turns, commencing close to the disc, after 
which the wire is carried on up the iron wire only, covering 
it for three and a quarter inches, reckoning from its attach- 
ment to the disc; both the four large turns and the smaller 
subsequent coil should be closely wound, a little separation, 
of about the thickness of one coil, being allowed between the 
last of the large and the first of the small turns. ‘This pre- 
vents the heat being too rapidly conveyed to the stage. The 
disc is now fastened with sealing-wax to a glass slide, the 
wire projecting beyond, while in the places where the labels 
are usually placed should be gummed slips of cardboard on 
both surfaces, so as to prevent the immediate contact of any 
part of the instrument with the stage of the microscope. 
It does not matter how the iron wire is bent; that here 
shown is the most convenient form for use with such a micro- 
scope as Verick’s. The next step is to graduate the warm 
stage thus constructed, and this once carefully done—and a 
