Professor Black Reproduces Foucault-Michelson Experiment in Determination of Speed of Light

Using methods patterned after the famous experiments of Foucault and Michelson, N. Henry Black '96, assistant professor of Physics, has been determining the speed of light by means of apparatus constructed under his direction in the Physics department. Professor Black pointed out that, in spite of its enormous size, the speed of light is known with greater accuracy than almost any other physical constant and that it is important in the study of many other types of radiation besides optics.

In Professor Black's experiment a beam of light from a small are is passed through a slit, through a half-silvered mirror, and onto a rapidly rotating stainless steel mirror. This sends the light down the length of a corridor in the Research Building, through a special longfocus lens, and onto a plane mirror which reflects it back to the rotating mirror and thence to the half-silvered glass. The opacity of this glass diverts part of the light beam into a microscope where the image of the slit may be watched closely. Because of its great speed, the rotating mirror moves an appreciable distance during the time that the light is travelling down and back the length of the corridor. It therefore causes a deflection of the image of the light slit, which may be measured accurately by means of the measured accurately by means of the microscope. From the deflection the speed of light is calculated.

The experiment so far has yielded results which are only accurate to within about five per cent. To obtain greater precision, Professor Black explained, the apparatus could not be nearly as suitable for demonstration to students. The most important part of the apparatus is the rotating mirror, which is driven by compressed air at the rate of 120 revolutions per second. Professor Black believes he will be able to raise this to one or three times that figure and there by obtain still more accurate results.