plane mirror on stand

Date1937-1941

Inventory Number1996-1-0676

ClassificationMirror

Subject

Maker Wilmer C. Anderson

User Department of Physics, Harvard University founded 1884

Cultural Region

Place of Origin

Dimensions30.3 × 30.5 × 15.5 cm (11 15/16 × 12 × 6 1/8 in.)

Material

DescriptionThe four-inch mirror, coated with chromium and aluminum, is centered in a larger circular mount. The latter finely pivots back and forth with the help of a screw located at the bottom of the arc supporting the mirror. The arc is itself supported by a heavy pedestal, which can finely move sideways using two screws (pushing on a vertical pin located between them). The mirror's fine motions were to better target the reflection of the light beam used in the experiment. Three clamps were also used to attach (a larger mirror?) on top of the smaller one.

In Collection(s)

Exhibit 2008--The Matter of Fact

Signedunsigned

Inscribedon the pedestal: CRUFT LABORATORY

Curatorial RemarksThis mirror, though it could have been used in the earlier experiments, is very similar to one found in the next experimental set-up Anderson devised to measure the speed of light. This new set-up, described in the 1941 article of the <i>Journal of the Optical Society of America</i> involved new types of Kerr cells, a new light source (a 1000-watt water-cooled mercury arc lamp) and changes in mirror supports. This mirror, shown in figure 7 of the article, is strikingly similar to this one. It was put on a lathe so it would move.

FunctionThis mirror was part of a larger experimental set-up devised by Wilmer C.
Anderson to measure the speed of light. According to the description in the Review of Scientific Instruments, the set-up consisted of a light source (a 500-Watt Point-O-Lite bulb), Nicol prisms, a Kerr cell (a photoelectric cell that replaced the human eye; it is really the innovation here), and mirrors. The present mirror, from Anderson's description, could be the one he identified as M5.

In the same article, Anderson said that after a series of 651 measurements, the mean result for the speed of light was 299,765 km/s ± 15 km/s. Two years later, he was able to get an even more precise number using 2895 observations: 299,776 km/s ± 14 km/s.

The origins of Anderson's work is found in his Ph.D. dissertation at Harvard University, completed in 1936 and entitled A High-Frequency Method for Measuring the Velocity of Light.

ProvenanceFrom the Department of Physics, Harvard University.