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THE CYCLOSTAT.

The various processes commonly employed for the observation of bodies in motion (intermittent light or vision) greatly fatigue the observer, and, as a general thing, give only images, that are difficult to examine.  We are going to show how Prof.  Marc Thury, upon making researches in a new direction, has succeeded in constructing an apparatus that permits of the continuous observation of a body having a rapid rotary motion.  The principle of the method is of extreme simplicity.

[Illustration:  FIGS. 1, 2, AND 3.—­DIAGRAMS EXPLANATORY OF THE PRINCIPLE OF THE CYCLOSTAT.]

Let us consider (Fig. 1) a mirror, A B, reflecting an object, C D, and revolving around it:  when the mirror will have made a half revolution, the image, C’ D’, of the object will have made an entire one.  The figure represents three successive positions of the mirror, distant by an eighth of a revolution.  The structure of the image shows that it has made a quarter revolution in an opposite direction in each of its positions.  But if (Fig. 2) the body itself has revolved in the same direction with an angular velocity double that of the mirror, its image will have described a circle in remaining constantly parallel with itself.  The image will be just as insensible as the object itself; but it is very easy to bring it back to a state of rest.

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.