Rotating, Mercury Mirror: Dr. R.W. Wood, Professor of Experimental Physics at Johns Hopkins University attempted in 1908 to make an automatically paraboloidal mirror of variable focal length by the theoretically practicable method of rotating on a central, vertical axis a round, shallow pan of mercury. Under centrifugal action the mercury takes on the figure of a true paraboloid. Using a 20 inch pan, a rubber thread transmission and a magnetic clutch, Dr. Wood obtained interesting results, the focal length being varied with ease by changing the speed. Minute irregular disturbances injured the perfection of the mirror's surface, despite the velvety transmission or drive. The mirror was rotated at the bottom of a well, and since it is horizontal, it reflected only the zenith stars; a flat would therefore be required to complete the equipment so that it would take in a large field. The original experiments were described by Dr. Wood in the Scientific American, March 27, 1909, page 240 (out of print -- consult at large public libraries), and in Astrophysical Journal, March, 1909. This interesting experiment was originally proposed in the Scientific American, Dec. 13, 1873, page 365 by someone who signed "D." It is known, however that "D" was David Todd, later to become Professor of Astronomy at Amherst College. Dr. Wood's experiment was not completed. The elimination of the ripples required a constant speed of drive.

The above note was submitted to Dr. Wood with a request for comment. He replied as follows: "The experiments were continued after the publication of the papers, but I never published anything more on it. I got it to work much better the second summer. I put a 20-inch flat over it and had excellent views of the Moon. The final conclusion was that constant speed of drive would eliminate the slight tidal wave, which was all that remained. I did not even have a synchronous motor. One of these, operated on a modern A.C. circuit with the cycle frequency controlled by clock, would be a great improvement. I do not advise anyone to try the mercury mirror, however."