Let us suppose (Fig. 3a) the observer placed at O, the revolving object at T, the axis of rotation being this time the line O F. Let us place a mirror at A B and cause it to revolve around the same axis; but, instead of looking at the image directly in the mirror, let us receive it, before and after its reflection upon A B, upon two mirrors, C D and D E, inclined 30 deg. upon the axis of rotation of the system; the image, instead of being observed directly in the mirror, A B, will always be seen in the axis, O F, and will consequently appear immovable. The same result may be obtained (Fig. 3b) with a rectangular isosceles prism whose face, A B, serves as a mirror, while the faces, A C and B D, break the ray--the first deflecting it from the axis to throw it on 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.]