92 On Methods of Microseopical Besearch. [^ouSl^ZTriff 
the stage by converting constant currents into heat recommends 
itseh" to us. In microscopic experiments only very small quantities 
of heat are required, and it is not at all necessary for the stage to 
be heated throughout its whole extent, but only its centre, or what 
is still better, a glass plate inserted in a caoutchouc plate. Such 
small quantities of heat might be expected from the circulation of 
even weak currents. It is known that the heating of a wire which 
forms part of the circuit of a constant battery increases as the 
thickness of this wire diminishes —according to Eiess, inversely as 
the square of its diameter. It is therefore only necessary to fix 
a correspondingly thin wire into the middle of a glass plate, put 
the two ends of the wire in communication with the electrodes of 
a constant battery, and close the current, and the glass plate gets 
heated. The cementing on of a wire is, however, inconvenient ; we 
have an excellent substitute in tinfoil. Thus I cut the tinfoil in the 
form of a picture-frame, with two arms projecting from opposite 
sides, gum it to a. slide, and connect the two ends of the tinfoil with 
the poles of the battery, and our object is attained. 
A very convenient method of connecting it with the battery is 
the following : — Brass springs are added to Hartnack's microscopes, 
by means of which the preparation can be held in a desired position. 
These springs, which are inserted by brass pins into hobs in the 
stage, I provide with caoutchouc pins; thereby they are isolated 
from the microscope. Whilst retaining the object-bearer in its 
place, they can at the same time press upon the broad ends of the 
tinfoil. I need then only affix a conducting-wire on any part of 
the spring on each side, and the circuit is closed by the tinfoil. A 
second strip of tinfoil of the same width as that affixed to the shde 
wound round the bulb of a thermometer, and inserted in any part 
of the circuit and suitably protected, indicates the temperature 
which the centre of it must have, if all secondary conditions are 
the same in both cases. These secondary conditions, however, may 
be met by the judicious use of the thermometer, which is necessary 
in all cases, according to whatever method the heating is carried on. 
A quantity of fat, the melting-point of which is known, should be 
put upon the place where otherwise the object would be placed, to 
ascertain the height of the column of mercury at the moment when 
the fat begins to melt. The fat should moreover be employed in 
pieces of microscopic size, and be watched through the microscope. 
It is best to cut a disc out of the fat, to cover it lege artis, to view 
it with a given base, and to make use of it for this lens. 
