which comprised a series of apparatus of the same class as those that
we have just enumerated--spyglasses for the reading of apparatus,
galvanometers, magnetometers, etc.

[Illustration: FIG. 3.--EXHIBIT OF TH. EDELMANN.]

Specially worthy of remark were the apparatus of Mr. Kohlrausch for
measuring resistances by means of induction currents, and a whole
series of accessory instruments.

Among the objects shown by other exhibitors must be mentioned Prof.
Von Waltenhofen's differential electromagnetic balance. In this, two
iron cylinders are suspended from the extremities of a balance. One of
them is of solid iron, and the other is of thin sheet iron and of
larger diameter and is balanced by an additional weight. Both of them
enter, up to their center, two solenoids. If a strong current be
passed into these latter, the solid cylinder will be attracted; but
if, on the contrary, the current be weak, the hollow cylinder will be
attracted. If the change in the current's intensity occur gradually,
there will be a moment in which the cylinders will remain in
equilibrium.

[Illustration: FIG. 4.--EDELMANN'S QUADRANT ELECTROMETER.]

Prof. Zenger's differential photometer that we shall finally cite is
an improvement upon Bunsen's. In the latter the position of the
observer's eye not being fixed, the aspect of the spot changes
accordingly, and errors are liable to result therefrom. Besides,
because of the non-parallelism of the luminous rays, each of the two
surfaces is not lighted equally, and hence again there may occur