How to use the Foucault Device
At this stage you will naturally be curious about the figure on the mirror, and will want to experiment with Foucault's lest. It is not essential that this test be conducted in total darkness, although delicate shadow contrasts are then more apparent.
Placing the mirror in the testing rack, find the reflection of the lamp window on the piece of ground glass or sheet tin as was previously done in checking the focal length, and by adjusting the rack, bring the sharply focused image to a position like that in Fig. 19.
Sit down with your eye about a foot in back of this point and, using a piece of ground glass, locate the position of sharpest focus. Bring the knife-edge up just tangent to
this image and in the same plane with it, as nearly as possible, at the same time dispensing with the ground glass. Now reach out and slip the band containing the pinholes down so that the large pinhole is over the window.
If your head is still about a foot back of the knife-edge, the tiny illuminated pinhole image should be seen superimposed on the mirror. By changing the focus of your vision from the mirror's distance to the usual reading distance, you will distinctly see the image of the pinhole suspended in space in line with the mirror and about a foot in front of your eye.
The convergent rays from the mirror intersect in this point (see Fig. 21a), then diverge, and it is somewhere in this divergent cone that the eye is now centered. Shift the focus of vision back to the mirror, and now, by bringing your eye forward along the axis of the cone, more of the divergent rays are collected by it, and the pinhole image appears to expand on the mirror's surface until, at a point where both the pupil of the eye and the divergent cone have the same diameter, the mirror appears fully and brilliantly illuminated. The distance of your eye from the knife-edge will then be about 21/2 " or more, depending on the size of the pupil.
But it would sorely try your patience to attempt to test from this position, as your head can hardly be held immobile, and the slightest movement might cause you to lose sight of the image. So your eye should be advanced to within about 1" of the knife-edge, where some latitude of motion will be enjoyed with less chance of losing sight of the reflection.
part 2 How to use the Foucault Device |
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