the mirror, and the second throwing it back to the axis of rotation,
which is at the same time the line of direction of the sight.

The principle of the instrument, then, consists in causing the
revolution, around the axis of rotation of the object to be observed,
of a mirror parallel with such axis, and in observing it in the axis
itself after sending the image to it by two reflections or two
refractions. In reality, the entire instrument is contained in the
small prism above, properly mounted upon a wheel that may be revolved
at will; and, in this form, it may serve, for example, to determine
the rotary velocity of an inaccessible axis. For this it will suffice
to modify its velocity until the axis appears to be at rest, and to
apply the revolution counter to the wheel upon which the prism is
mounted, or to another wheel controlling the mechanism.

But Mr. Thury has constructed a completer apparatus, the _cyclostat_
(Fig. 4), which, opposite the prism, has a second plate whose
actuating wheel is mounted upon the same axis as the first, the
gearing being so calculated that the prism shall revolve with twice
less velocity than the second plate. This latter, observed through the
prism, will be always seen at rest, and be able to serve as a support
for the object that it is desired to examine.

[Illustration: FIG. 4.--THE CYCLOSTAT.

1. General view of the apparatus.
2. Section of the ocular, O.]

The applications are multitudinous. In the first place, in certain
difficult cases, it may serve for the observation of a swinging