mirror on stand

Date1937-1941

Inventory Number1996-1-0664

ClassificationMirror

Subject

Maker Wilmer C. Anderson

Cultural Region

Place of Origin

Dimensions43 × 40.5 × 40.5 cm (16 15/16 × 15 15/16 × 15 15/16 in.)

Material

DescriptionSilvered mirror on a heavy tripod stand. Each leg has a leveling screw. The position of the mirror can be adjusted and rotated in all three axes by using micrometric screws. An inscription taped on one leg mentions: "Danger / do not touch Mirror".

Signedunsigned

Curatorial RemarksIt is unclear whether this mirror was used in the first, second, or both sets of experiments by Anderson to determine the speed of light. It is very similar to one found in the second experimental set-up described in the 1941 article of the <i>Journal of the Optical Society of America</i>, which involved new types of Kerr cells, a new light source (a 1000-watt water-cooled mercury arc lamp) and changes in mirror supports.

FunctionThis type of 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.

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.