Foucault Tester, Continued

Page 2


I used a 3/8" rod for the mechanism to slide on per Richard Berry's book. It is seen on the left in the photo below. Drill holes in 2 blocks for the rod to pass through. They needn't be super tight, but snugness is helpful.

Drill another hole in a third block for the 1/4" screw, which adjusts the lateral travel. This is a carraige bolt with the black knob in the photo below. A rubber band pulls the top slider against the travel screw. The travel screw is held in the wood block by a 1" long nut (obtainable at most hardware stores). The long length helps ensure that slop is minimal in the screw. Some have threaded the rod into a soft plastic like teflon instead of using wood with the nut.

Below is the completed unit, minus the measuring dial, which goes at the end of the screw near the black knob. The screw which sticks out vertically in the top board is used to tilt the blade into the light path. You could add a knob to it, but it's easy to turn and I never bothered. How much it moves is not measured, so you don't need anything fancy. It just needs to smoothly move into the light path so you can adjust the shadows on the mirror to get equal grays on opssoite sides of the mirror with a Couder mask (or stick pin).

If you have lots of questions (or even just a few) you may email me at

NEW! More photos, showing scale and optional micrometer

Good luck building!

More detailed instructions on building and testing are available with our software.

To see some actual sample photos while using this tester, see...

Test Images