SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY • NUMBER 37 

Science and the Instrument-maker 

MICHELSON, SPERRY, AND 

THE SPEED OF LIGHT 

Thomas Parke Hughes 

Qmit/isoDian Ij;i^titution 'PJ^SS 

City of Washington 

1976 
ABSTRAC T 

Hughes, Thomas Parke. Science and the Instrument-maker: Michelson, Sperry, 

and the Speed of Light. Smithsonian Studies in History and Technology, number 

37, 18 pages, 9 figures, 2 tables, 1976.-This essay focuses on the cooperative 

efforts between A. A. Michelson, physicist, and Elmer Ambrose Sperry, inventor, 

to produce the instr.umentation for the determination of the speed of light. At 

the conclusion of experiments made in 1926, Michelson assigned the Sperry instruments the highest marks for accuracy. The value of the speed of light accepted 

by many today (299,792.5 km/sec) varies only 2.5 km/sec from that obtained 

using the Sperry octagonal steel mirror. The main problems of producing the 

instrumentation, human error in the communication of ideas to effect that instrumentation, a brief description of the experiments to determine the speed of 

light, and the analysis and evaluation of the results are discussed. 

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded 

in the Institution's report, Smithsonian Year. Si PRESS NUMBER 6I41 . COVER: A. A. Michelson 

and Elmer Ambrose Sperry. 

Library of Congress Cataloging in Publication Data 

Hughes, Thomas Parke. 

Science and the instrument-maker. 

(Smithsonian studies in histoi7 and technology ; no. 37) 

Supt. of Docs, no.: SI 1.28.37 

1. Light—Speed—Measurement. 2. Michelson, Albert Abraham, l852-l93l. 3. Sperry, Elmer 

Ambrose, 1860-1930. I. Title. II. Series: Smithsonian Institution. Smithsonian studies in 

history and technology ; no. 37. 

QC407.H83 535'.24 76-619433 

For sale by the Superintendent of Documents, U.S. Government Printing OfRce 

Washington, D.C. - Price 65 cents (paper) 

Stock No. 047-000-00341-5 
Contents 

Page 

Introduction 1 

Background for Cooperation 2 

The Men 2 

Sperry's Searchlight 3 

Determination of the Speed of Light 4 

Nature of the Experiment 4 

Trials of Instrumentation 5 

Analysis and Evaluation of the Data 12 

Appendix 15 

Notes 17 

Ul 
Science and the Instrument-maker 

MICHELSON, SPERRY, AND 

THE SPEED OF LIGHT 

Thomas Parke Hughes 

INTRODUCTION 

Science is popularly conceived of as a cerebral 

activity. The image of Einstein superimposed upon 

a background of esoteric notation dominated by the 

formula E = mc^ has become an item of popular 

culture. However, many scientists—especially experimental as contrasted with theoretical physicistswould be among the first to acknowledge their reliance upon instruments and, recently, big machines 

like particle accelerators. In many instances the 

physicists have designed and even built their own 

apparatus, but in many others they have depended 

upon the skilled instrument-maker and engineer. 

Only infrequently has the public been made aware 

of the role of these craftsmen, and many of their 

names have been lost to history. 

This essay * tells of a distinguished scientist's dependence upon the mechanical ingenuity of the 

maker of instruments. After World War I Albert 

Abraham Michelson, America's first Nobel prize 

winner, embarked upon a renewed attempt to 

establish the speed of light. For this endeavor he 

used a greatly improved light source made by the 

Sperry Gyroscope Company, which was established 

by Elmer Sperry, one of America's most famous 

twentieth-century inventors. 

• I appreciate comments from Preston 

Cooper, D. Theodore McAllister, and R. S. 

early drafts of this essay. 

Bassett, Leon 

Shankland on 

Thomas Parke Hughes, Department of History and Sociology 

of Science, University of Pennsylvania, Philadelphia, Pennsylvania 19174. 

Pleased by the results with the searchlight, 

Michelson again, in 1924, turned to Sperry for 

another instrument, a high-speed revolving steel 

mirror. The mirror, like the searchlight, was a critically important component in the experimental 

system designed by Michelson to measure the speed 

of light. His resorting to Sperry for a high-speed, 

multifaced, steel mirror was quite appropriate, for 

the Sperry Gyroscope Company was the manufacturer of highly precisioned devices, such as a gyrocompass (invented by Sperry) with its high-speed, 

perfectly balanced rotor. When Michelson sought 

assistance, the mechanical skill and the precision 

machine tools used to make gyrocompasses at Sperry 

Gyroscope were enthusiastically committed to a 

challenging pure-science problem. In providing 

Michelson with the searchlight and, later, the steel 

mirror, Sperry, his engineers, and master machinists 

were playing the role that scientific instrumentmakers had assumed since the 15th century for such 

observers of natural phenomena as Galileo Galilei, 

Tycho Brahe, and J. B. Leon Foucault. The instrument-makers' exhilaration of contributing to and 

participating in the search for abstract truth on a 

grand scale still stirred the imagination of Sperry 

and his employees. This cooperative spirit between 

the instrument-maker and the scientist was expressed most positively by Sperry, who—after learning of how well the revolving mirror performed— 

wrote, "to think that we have been in any way helpful in this great work fairly takes our breath 

away." ^ 
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

Due to human fallibility, however, the quest for 

precision, even perfection, by all those involved— 

but especially Michelson—suffered many a setback. 

Dedicated to the determination of what was thought 

to be an eternal constant (the velocity of light), a 

fundamental needed by physicists, astronomers, 

navigators, and in the Einstein equation (E = Tnc^), 

Michelson was attempting to reduce the margin of 

computational error from about 100 to one or two 

kilometers in several hundred thousand. But ironically, his specifications regarding the instrumentation were so vague that the first model built from 

them proved useless. Other human "frailties" also 

threatened to add their bias to the determination 

of the speed of light. 

It was necessary, for example, to ship the mirrors 

from their place of manufacture, Brooklyn, New 

York, to California. The freight handlers, insensitive to the prize they bore, delivered one mirror in 

such a lamentable condition that Sperry had to seek 

new methods of packing to prevent damage in 

transit. To compound the confusion, Michelson, 

after he had a properly functioning mirror, reported one set of data privately to Sperry and published another. Furthermore, the published results 

contained discrepancies and raised questions about 

Michelson's judgments, or evaluations. All of this 

is a part of the story of frail men striving with instruments of the highest obtainable precision to 

establish truths far removed from the uncertainties 

of a world—including freight handlers—in which 

they live and work. 

BACKGROUND FOR COOPERATION 

The Men 

By the mid-twenties, Elmer Ambrose Sperry 

(1860—1930), inventor and engineer, was world 

famous for his applications of the gyroscope for 

guidance and control of ships and airplanes. The 

Sperry Gyroscope Company of Brooklyn, New York, 

supplied the world's navies with gyrocompasses, and 

the company had an established reputation for the 

precision manufacture, as well as for research and 

development, of these tools. Skilled machinists, 

clever young design engineers, and the ever-resourceful and widely experienced inventor and 

engineer, Sperry, gave the company a unique character. Guided by Sperry's known preference for 

difficult problems, the company was noted for the 

kind of technology the mass manufacturers would 

not attempt.2 

Albert A. Michelson (1852-1931), America's first 

Nobel laureate (1907), fully appreciated the dependence of his experiments on highly precisioned 

instrumentation. His Nobel Prize was awarded "for 

his optical precision instruments and the spectroscopic and meteorolpgical investigations carried out 

with their aid." ^ Michelson's eminence, especially 

his reputation for instrumentation, stemmed not 

only from his velocity of light determinations, but 

also from his series of experiments carried out from 

1881 to 1927 on another fundamental problem: the 

effects of relative motion on the velocity of light. 

The instrument he had designed for this work, the 

inferometer, has been called "a lovely thing." * 

Michelson, famous for this and his other instruments, came "to regard the machine as having a 

personality . . . almost .. . a feminine personalityrequiring humoring, coaxing, cajoling—even threatening! [sic]" ^ 

Michelson's attitude toward his instruments was 

shared by Sperry, who also personified machines, on 

occasions calling them "little fellows," and, at more 

exasperating times, "brutes." ® Sperry had an affection and respect for complex, high-quality machines 

and instruments and for the fine machinists and 

craftsmen who made them.'From his beginnings as 

an inventor, he depended upon the forge and machine shop to fulfill his inventive concepts. In 

order to have his first invention built, he helped 

in the shops of the Cortland (N.Y.) Wagon Company, his home town's leading industry. He learned 

then, that an experienced machinist could modify 

his designs so that the potential of his inventions 

could be most fully exploited. In 1910, after Sperry 

formed the Sperry Gyroscope Company, which 

grew to employ t^iousands of workers, he continued 

to "talk out" with his head machinist the design 

of a new device, rather than to hand down drawings and specifications. When his company was 

young, its capital assets were simply Sperry's patents 

and its machine tools. Sperry's son, Elmer, Jr., 

attributed the success of the company in discour-
NUMBER 37 

aging competition to the fine Swiss machine tools 

and German machinists at Sperry. 

Another common bond between the scientist 

Michelson and the engineer Sperry was the pioneering work of J. B. L^on Foucault (1819-1868), upon 

which they both drew. Sperry's major gyrocompass 

patent, appHed for in June 1911,^ begins with 

acknowledgement of his indebtedness to Foucault 

for his basic ideas about the gyrocompass. Michelson, for his first determination of the velocity of 

light in 1877, used apparatus based upon a minor, 

but critical, modification of the revolving mirror 

technique employed by Foucault.^ Foucault had 

employed a revolving mirror in 1862 to determine 

the velocity of light. Earlier, in 1850, he had used 

the mirror to establish whether light traveled with 

greater or lesser velocity in a refractive medium, 

such as water, than it did in air.® Foucault also set 

a precedent for Sperry. In 1852, Foucault used a 

freely suspended gyroscope with its orientation fixed 

in space to demonstrate the relative motion of the 

earth. Then—and this later proved helpful to Sperry 

—Foucault showed how the gyroscope could be 

modified (weighted, or made pendulous) and used 

as a compass indicating true north, as it aligned its 

axis of rotation with the axis of the earth. 

A two year period of cooperation between Sperry 

and Michelson began in 1924, when Michelson 

asked the engineer to supply him with a steel revolving mirror to use in his new series of velocity 

of light experiments begun in 1921. The request for 

a Sperry steel mirror was probably stimulated by 

the good impression that a Sperry searchlight, already put to use in the new experiments, had made 

on Michelson. It is not unlikely that the scientist 

had first decided upon a new determination when 

he learned of the remarkable improvements of 

searchlights during World War II,.for a more intense light source would allow the experimenter to 

use a longer base line over which the light to be 

measured would travel. Before following the story 

of the steel revolving mirrors, therefore, a brief 

account of the Sperry searchlight is in order. 

Sperry's Searchlight 

In 1914, the United States Navy had tested a 

high-intensity searchlight patented by a German 

inventor, Heinrich Beck, and found it to have an 

illuminating capacity five times that of the light 

then standard on Navy ships. With the war in 

Europe increasing the Navy's interest in preparedness, it contracted with General Electric for the purchase of Beck lamps, which the electrical manufacturer was making under the Beck patents. Sperry, 

familiar with the Beck lamp, saw several ways to 

improve it. Arc lighting was not a new field for 

Sperry who, when he was only twenty-one, had 

patented an automatically regulated arc lamp in 

1881. Not only did Sperry believe he could improve 

upon the Beck lamp and obtain patents on the improvements, but he knew that his close contacts 

with the Armed Forces resulting from his invention and manufacture of sophisticated gyroscopic 

and fire-control devices would obtain for him a good 

hearing and a fair trial of his own lamp. By 1916 

he was ready to have his searchlight entered into 

competitive trials. He had been assisted in developing the device by several members of his staff of 

engineers, among them Preston Bassett, a young 

chemistry graduate from Amherst College. (Later, 

Bassett had the privilege of demonstrating the 

searchlight for Michelson.) 

The improvements made by Sperry, Bassett, and 

others upon the Beck lamp included the use of air 

instead of alcohol for cooling the electrodes in order 

to keep the flame, or arc, at the tip of the electrodes. 

The result was a more intense light. In substituting 

air for alcohol, Sperry simplified the searchlight 

mechanism, a technique (simplification) he frequently resorted to when he found himself competing with German inventors-who tended to be perfectionists (and unrealistic, Sperry would add). 

Sperry also introduced a different means for feeding the electrodes to maintain the arc gap as they 

burnt down. He also used a different way than Beck 

for positioning the positive electrode so that the 

concentrated arc would be at the focal point of the 

reflector mirror of the searchlight. Preston Bassett 

at the same time worked upon the design and chemical composition of the electrodes. During World 

War I, with support from the U.S. Army, Sperry 

and his engineers further improved the design and 

manufacture of the searchlight for coast defense, 

anti-aircraft, and use in the field. After the war, 

Sperry adapted the searchlight for peacetime purposes by promoting its use as a beacon for guiding 

airmail planes on their lonely night flights across 

country, and adapting the lamp mechanism for use 

in movie projectors.^" 
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

Michelson utilized the Sperry searchlight for his 

velocity of light experiments before asking the inventor for a revolving mirror. However, Michelson's experimental team seems not to have fully 

understood the operation of the lamp. After it had 

been used in the experiments for a time, Bassett 

found that the lamp was being operated without 

the automatic control for positioning the electrodes, 

and he showed the Michelson staff how to use it 

properly. Nevertheless, Francis G. Pease, Michelson's assistant at the Mount Wilson Observatory in 

Pasadena, California, had informed Sperry, after 

trials in 1924, that "brightness of the arc gave him 

all he desired in the way of light and 'then some' 

and the rough preliminary results are better than 

any heretofore." ^^ No wonder that Michelson 

turned to Sperry, the inventor and manufacturer 

of complex, precision, technological devices, when 

Michelson encountered a serious problem with one 

of the instruments in his experimental system. 

DETERMINATION OF THE SPEED OF LIGHT 

Nature of the Experiment 

The design of Michelson's experiment can be described briefly in order to define better the problem 

encountered in the summer of 1924.^^ xhe apparatus of 1924 differed in particulars from that 

used by Michelson between 1878 and 1882 when he 

made his first determinations of the speed of light. 

However, the essential principle was the same: 

measure the time elapsed as light travels from a 

source out to a reflecting mirror and returns to the 

place of origin. This was also the essential means 

used for determination by Foucault in 1862, A. 

Hippolyte L. Fizeau in 1849, and Marie A. Cornu 

in 1872.^^ Michelson's 1924 apparatus differed from 

the earlier ones in the more accurate measurement 

of the time elapsed, the greater distance over which 

the light traveled, and, as noted, in the power of 

the light source used, which made possible the sending of the light over a greater distance. In the experiments of Foucault and Michelson before 1924, 

the experimenter determined the elapse of time by 

measuring the angle through which a revolving mirror rotated during the interval it took light to travel 

over a precisely measured distance. The angle of 

rotation was measured by noting the displacement 

of the beam of light by the rotating mirror. A stationary mirror would cause no displacement. The 

amount of displacement depended upon the velocity 

of rotation of the mirror, the distance traveled, and 

the speed of light. Because the first two could be 

measured the last could be calculated. The apparatus of Fizeau and Cornu, by contrast, employed 

a revolving gear wheel. The beam of light that 

passed between the teeth of the wheel on its outward journey was interrupted by the succeeding 

tooth of the revolving gear, resulting in a flash to a 

reflecting mirror at a precisely measured distance. 

When the gear wheel was revolved at a particular 

velocity—by the experimenter—then the flash passed 

through the next opening between the teeth upon 

its return. Knowing the distance traveled and the 

velocity of the gear wheel, the experimenter calculated the speed of light.^* 

In 1924 Michelson's apparatus employed a revolving mirror in the manner of Foucault and his 

own earlier experiments, but the new mirror was 

multifaceted so that Michelson could use it in a 

manner similar to that of Fizeau's gear wheel. 

"The advantage of the octagonal revolving mirror . . . ," he explained, "lies in the possibility of 

receiving the return light on a succeeding face, thus 

eliminating the measurement of the angular deflection of the returned beam. . . ." ^^ The accuracy 

now depended upon the construction of the octagon, a responsibility that fell to the instrumentmakers, including Sperry, his engineers, and machinists. 

Michelson introduced slight variations in the 

arrangement of his system during the period 1924 

to 1926, when he was conducting the velocity observations.^^ When making his definitive measurements using the Sperry apparatus, the system 

(Figure 1) included a multifaceted rotating mirror, 

an arc lamp (S) casting its beam through a narrow slit, reflecting mirrors (&,c,&i) to direct the 

beam of light along a desired path, a reflecting concave mirror (D), and a prism to direct the beam 

of light into the observing eyepiece (O). All of the 

foregoing apparatus was on Mt. Wilson. Twenty 

miles away on Mt. San Antonio was a concave mirror (E) and a reflecting mirror (f). The light path 
NUMBER 37 

was—as shown in Figure 1—from the surface a of the 

revolving mirror to bcDEfEDcb^ and to surface a' of the revolving mirror. B^ and B on the 

figure are bench marks for measuring distance. The 

slit through which the arc lamp beam passed was 

extremely narrow (0.5 to 0.1 mm) ^^ so that the 

location of its return image on the eyepiece could 

be observed with precision. 

'--V\ 

C ^ : SH::::> 

MtSmAntonio IW Mt.Wfilson 

FIGURE l.—Arrangement of Michelson apparatus. (From A. A. 

Michelson, "Measurement of the Velocity of Light between 

Mount Wilson and Mount San Antonio," The Astrophysical 

Journal, volume 65 (1927), page 4.) 

For his definitive experiments in 1926, Michelson 

used revolving mirrors of 8, 12, and 16 facets. The 

angular velocity imparted to these was such that 

the mirrors turned through a distance equal to the 

distance between the centerlines of adjacent facets, 

while the light was traveling out from Mt. Wilson, 

to Mt. San Antonio, and back (about 0.00023 seconds) . After Michelson adjusted the rate of revolution, the rotating mirror presented "the succeeding 

face of the mirror to the returning beam at (very 

nearly) the same angle as at rest." ^^ In conjunction 

with a prism, the eyepiece was positioned to observe 

the beam as it was reflected off the facet. When the 

light beam returned to the observer's eyepiece, he 

then knew that the mirror was revolving at the rate 

that would turn the mirror one-eighth, one-twelfth, 

or one-sixteenth of a turn (according to the revolving mirror used) during the time the light beam 

passed over the measured course. Extremely small 

angular deviations of the return image from the 

centerline were measured in order to ascertain 

precisely the angular distance moved by the mirror 

during the travel of the beam of light. 

Measurement of the distance between stations 

and the measurement of the speed of rotation of 

the revolving mirror also had to be determined 

with precision. The United States Coast and Geodetic Survey established the length of the light path 

with a result estimated to be accurate within one 

part in two million.^^ During the definitive observations, the angular velocity of the mirrors was 

regulated by a stroboscopic comparison of the revolving mirror (528 rps for the octagonal mirror) 

with an electric fork vibrating at a fixed rate (528 

vibrations per second for the octagonal mirror). 

The rate of the vibrating fork was controlled by 

comparisons with a free seconds pendulum which 

was compared with "an invar gravity pendulum 

furnished and rated by the Coast and Geodetic 

Survey." ^o 

Trials of Instrumentation 

Early in his experiments, Michelson encountered 

a serious problem with an octagonal glass mirror. 

When run up to speed, the relatively large mirror 

flew to pieces. Hence, his requests to Sperry for a 

mirror of steel. Michelson specified that it had to 

withstand a peripheral speed of 27,500 feet per minute (540 turns per second), a diameter of 3 inches, 

and a face of about 1 ^4 inches.^^ The mirror was to 

be rotated on a vertical axle, driven by an electric 

motor or by an air blast upon turbine blades. 

Michelson considered using an electric motor in 

order to operate the mirror in a vacuum. He asked 

if it would be necessary to provide electric motors 

both above and below in order to have both positive and negative rotation. His remark about positive and negative rotations only in conjunction with 

motors would prove ambiguous and unfortunate. 

Sperry accepted the assignment, confident that 

his company could do the engineering and drafting, 

make the patterns, perform the machine work, and 

take care of the high-speed dynamic and static balancing. "Our investigation shows," Sperry wrote, 

"that the little apparatus you require is no exception to the rule of machines that have to be made 

with a high degree of precision. As time goes on 

we find that our gyro compasses are no exception to 

this rule either." ^^ Louis Malkovsky, Sperry's brilliant master mechanic, who later became company 

vice-president for manufacturing, also accepted the 

problem enthusiastically, certain that he could machine and dynamically balance the high-speed 

rotor.23 

Sperry took pride in the assignment. Of a younger 

generation than that of Thomas Edison's, Sperry 

and his engineers eagerly associated themselves with 

pure science, while Edison's indifference—even 
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

animosity—seemed to increase. Sperry shared his enthuasism—and information about the importance of 

his company's contribution—with Professor Hantaro 

Nagaoka, called the "Lord Kelvin of Japan." 

(Sperry was greatly admired in Japan, not only for 

his invention, but also because he symbolized the 

practical application of science.) 

Did I tell you, that all of Dr. Michelson's tests in his new 

effort to refine the accuracy of the determination of the 

velocity of light was done with the Sperry arc? He told the 

great scientific gathering at the time of the Franklin Institute 

Centenary that the work would have been entirely impossible 

without this light. I was so surprised and embarrassed when 

he mentioned this that I could almost have sunk through 

the floor. He went on to say that with my arc he could easily 

have carried out the experiment on a base line of lOO miles. 

You probably heard about the rotor that ran some 530 

revolutions per second and exploded. We are now making 

him an entirely new apparatus here at our works and will 

have it under test in a couple of weeks. In this we use steel 

as the eight-sided mirror, about 3 inches in diameter at the 

corners. . . He expects to continue the experiments this 

June.=^ 

In April 1925, eight months after Michelson's 

request, Sperry reported the mirror ready. "We all 

feel very happy over the outcome of this device," he 

wrote, and "we have it in such a complete state of 

balance, both dynamic and static, that we can find 

no evidence whatever of vibration. . . ." The mirror 

was air driven because Michelson decided to keep 

the cost down; an electric motor would have to have 

been especially built. Sperry hoped that the machining of the facets was so fine that little polishing 

would be necessary, for polishing might disrupt the 

balance. Because there was a tendency toward heating within the casing and around the upper bearing, the casing was provided with a jacket for air 

cooling.^^ 

Ready for shipment, the apparatus was a handsome example of highly skilled craftsmanship 

(Figure 2). Beneath the mirror, a single row of 

turbine buckets received the air jets from four 

nozzles projecting from a ring casing. Air at 65 psi 

had been found to drive the mirror at about 600 

rps. In the casing, two diagonally opposed windows 

permitted the light beam to be directed upon one 

facet of the mirror and to be observed reflected 

upon another.2^ External fixtures provided for air 

to drive the mirror and cool the casing. 

The apparatus—so carefully and tenderly madereached Michelson in sad condition. It had not been 

FIGURE 2.—The 8-sided Sperry mirror with single row of 

turbine buckets. A part of the housing, including the ring 

with air nozzles, is also sho%vn. (Courtesy Michelson Museum, 

Naval Weapons Center, China Lake, California) 

disassembled in the Sperry packing room, as 

directed, and it arrived with the upper end of the 

shaft tightly imbedded in the shipping crate. As a 

result, the shaft was sprung several hundredths of an 

inch. Furthermore, Michelson noted that the mirror 

surfaces did not appear to be hardened by heat 

treatment (nor had he so specified), and he doubted 

that unhardened mirror facets could be optically 

polished. The instrument was returned "via American Express." ^^ 

Alexander Schein, a Sperry engineer, assuming 

responsibility during Sperry's temporary absence, 

thought Michelson wrong in asking that the 

octagonal surfaces of the rotor be hardened, so that 

they could be optically polished to obtain the mirror surface. Schein believed that the nickel steel 

used could be lapped very fine to a mirror-like condition without hardening, but he instructed the 

shop to proceed immediately with comparison tests 

of the brilliancy of unhardened and heat-treated 

mirror surfaces.^^ The test proved Michelson correct. Sperry, upon his return to the city, promised 

to give Michelson the "maximum hardness, which 

works about 85 on the scleroscope." ^^ By heating to 

about 1600° in an electric furnace by 40° to 50° 

steps and by various quenching techniques, the surface was brought up to glass hardness.^" Despite successful hardening, Michelson later observed that the 

lack of homogeneity of the steel surface made figuring and polishing steel more difficult than glass. 
NUMBER 37 

The misunderstanding concerning hardening resulted from the lack of detail in Michelson's original specifications.31 Michelson cheerfully admitted 

"that he had not been sufficiently definite" on the 

hardening.32 Sperry responded by admitting that he 

had been mistaken in assuming that nickel steel did 

not need hardening for the polishing.^^ 

The lack of specific instructions led to other 

wrong turns and backtracking by the distinguished 

inventor and the Nobel laureate. For instance, 

Sperry took the request for windows in the housing 

as one for "openings," and he initially advised 

Michelson against glazing or putting glass in them. 

He reasoned as follows: 

Now, do you really need any provision for glazing these 

windows? You understand that there are no oil fumes or 

anything of that kind coming out of this structure. The 

only thing that will manifest itself is the fan action of the 

eight sides of the mirror, and inasmuch as your eye is not 

near the window and no observation is made close up, I 

don't see why two plain openings are not all that you require. We take it that these two windows must be exactly 

opposite each other, but if they need to be at any particular 

angle, you will of course have to tell us what angle. The 

main thing we want to know is, won't a plain opening do? 

The only difference that we can think of here with our 

experience in running the thing is that with the opening on 

the exterior of this drum it will have a slight blower action, 

and therefore require a little higher air pressure to maintain 

the same speed. This, however, will only be a small item.^ 

Several weeks later, however, experience proved 

a truer guide than reason, and Sperry cautioned 

Michelson: 

We are now writing you, principally to urge you not to 

think of running this job without the windows because of 

the enormous handling of air and the increased pressure 

required to operate. Moreover, large suctions are produced 

in the blower actions which tend to suck out all the oil 

from the bearings and put them in inoperative condition by 

drying them up in a very short time, but above everything 

is the terrible screech of the machine, acting as a siren. I 

think this would soon get on the nerves of anybody within 

half a block. It is astonishing how penetrating this noise is. 

You see the note that it makes is about 4300 a second and 

way up 'in the top of your head.' You are so familiar with 

optical glass and our light source is so powerful that I 

believe you will find it all right to run with these windows 

closed, as only an extremely thin film of glass is required 

over the openings.*' 

On 27 May, the "beautifully running" instrument was repacked in "April fool" packages (one 

box within another) and sent to Michelson, who 

was advised, "You will finally find the little device 

somewhere inside." ^^ Sperry offered to contribute 

the device, if payment was to be from Michelson's 

personal account, for Sperry begged "the honor of 

considering this apparatus as my little contribution 

[c. $400] towards your wonderful work along the 

line of giving the astronomer greater accuracy as 

to his yardstick." ^^ 

Michelson, aged 73, was ill in the summer of 1925 

and was not able to inspect the instrument until 

late August. "Much to my chagrin and owing to an 

oversight which I trust your generosity will attribute to my illness," Michelson wrote, "I find it necessary to return to you again the revolving mirror . . . ." ^^ It had been provided with only one row 

of turbine buckets (Figure 2) ; two rows were 

needed to allow for a counter air blast to adjust 

speed and for rotation in the negative and positive 

directions (Michelson did not—or did not choose 

to—recall his penciled letter of 23 August 1924, to 

Sperry in which he asked for both positive and 

negative rotations, but only in connection with an 

electric motor drive) .^^ Sperry, a veteran of countless mishaps during decades of invention and development, replied: "As to the little mirror, my 

dear Michelson, you know that your merest desire 

is our command, so that the moment it gets here 

we will see just what is necessary to accomplish 

what you wish. . . ." ^° 

Considering Michelson's work as an exciting 

challenge, Sperry, his engineers, and technicians did 

not lament the extra effort and expense. Preston 

Bassett, who headed the Sperry searchlight department and later the Sperry Gyroscope Company, expressed their sentiments well after visiting the Mt. 

Wilson Observatory to overhaul and adjust the 

Sperry searchlight. Bassett spent three days there 

working with Michelson's assistant, Fred Pearson, 

and he was thrilled to see the Sperry light reflected 

from "Old Baldy" 22 miles away. "I doubt," the 

young engineer wrote, "If I ever enjoyed three days 

as much as these." ^^ Later, in a similar vein, Sperry 

told Michelson: 

The superintendent of our grinding department has personally spent more than a week in continuous work on the 

final grinding of the facets on the mirrors and, incidentally, 

I might confide in you that he is nearly a wreck as a result. 

Our boys have all become so intensely interested in the 

production of these mirrors that they want to know what 

it is that you do next and how you make the final determinations of accuracy. We have gone up, Mr. Schein and I 

believe, almost beyond the accuracy possible with any in-
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

struments that we can obtain or money can buy in this 

country, so you don't know how much I would appreciate 

some little word from you that I could pass on to the boys, 

giving us an insight into the farther history of these two 

exotic plants that have been brought forth and nurtured 

with the greatest labor and the tenderest care." [Figures 

3,4.] 

After the instrument with the single row of turbine buckets was returned, Michelson inquired 

FIGURE 3.—The film sequence indicates the precision work 

done at Sperry Gyroscope Company on the rotors of gyrocompasses. These expert craftsmen and highly developed 

precision machine tools made the revolving mirrors for 

Michelson. a, "The rotor, after being heated, is shrunk on 

the armature shaft." b, "The rotor must be accurately balanced to eliminate all vibrations. It is suspended on a 

flexible cable and spun at 6000 r.p.m. The slight touch of 

the balancer's pencil marks the high sides of the shaft, 

detecting the most minute vibrations." c, "A little metal is 

drilled from the heavy side. This process is repeated until 

the rotor is perfectly balanced." (From The Sperryscope, 

volume 3, number 12 (1923), page 13.) 

about a 12-faceted mirror, as well as an octagonal 

one. Michelson and Sperry agreed that the two 

mirrors could be interchangeably operated in the 

same housing. When Michelson had expressed concern about costs, Sperry replied: 

You ask for an estimate. The boys have gone through the 

costs very carefully and of course you can appreciate that the 

experimentation on the first one ran the cost very high. 

That experience, however, will help to cut the cost of the new 

apparatus. You may be interested to know that the hardest 

job about the whole matter is grinding the surface with the 

accuracy that we secured before and getting a running 

balance at the extremely high speeds, but having had experience with the other, we are hoping that it will help us 

with the new one. 

We should like again to make a financial contribution 

towards this matter, but I do not think you wish us to make 

as great a one as before. The price we are quoting does not 

cover the entire cost, but we are prepared to build the two 

reversible mirrors which would fit the same casing and 

strive for about the same accuracy that we had before 

throughout, for $665. You understand that this includes the 

8-sided revolving mirror which will run in both directions 

and also another l2-sided mirror which will run in the two 

directions, both organized to operate in the present modified 

casing and on the same journals. We are happy to say that 

we have found a way whereby this can be done very nicely.** 

Michelson wired two days later that he would like 

to know the price of the reversible 8-sided mirror 

omitting the order for the 12-sided one. Sperry telegraphed immediately, "Will you kindly permit me 

to donate double eight-sided and twelve-sided 

equipment to the great scientific work you are 

doing? ** 

Three months were needed at the Sperry company to complete the work, which was ready late in 

January 1926. On the eve of shipment, Sperry reported that the 8-sided mirror had been run up to 

almost 600 rps with 60 psi and the 12-sided to the 

same speed with 40 psi. The balancing was so well 

done that there was no apparent evidence that 

there was motion in the case. The two mirrors operated equally well in either direction (Figures 

5-9) .*^ In a formal letter of appreciation entered 

by Michelson in the record of the Board of Trustees 

at the University of Chicago, Michelson's university, 

he acknowledged Sperry's gift of "two beautiful 

specimens of workmanship" and the loan of "a 

magnificent arc-light" to which Michelson attributed "a large measure of the success of last year's 

[1925] work on Mt. Wilson." « 

Observations using the Sperry mirrors—and glass 
NUMBER 37 

FIGURE 4.—Scenes from the general machine shop at Sperry Gyroscope Company where machinist-artisans made precision devices in the tradition of scientific instrument-makers: a, jigborers section; b, multiple-drill presses section. 
10 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

"W;f%.-^-

FIGURE 5.—The 8-sided Sperry mirror with double row of 

turbine buckets. (NMHT 3l9,992, Smithsonian photo 59416) 

FIGURE 6.—The 12-sided Sperry mirror. (Courtesy Michelson 

Museum, Naval Weapons Center, China Lake, California) 

FIGURE 7.—The 12-sided Sperry mirror with housing disassembled. (Courtesy Michelson Museum, 

Naval Weapons Center, China Lake, California) 
NUMBER 37 11 

FIGURE 8.—The l2-sided Sperry 

mirror with housing and mounting. (Courtesy Michelson Museum, Naval Weapons Center, 

China Lake, California) 

FIGURE 9.—The 12-sided Sperry 

mirror assembled. The gauge 

measures the air pressure that 

drives the mirror; other air 

intake is for cooling. (Courtesy 

Michelson Museum, Naval Weapons Center, China Lake, California) 
12 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

mirrors—were made in the summer of 1926. Upon 

his return from Mt. Wilson on 22 September, 

Michelson wrote Sperry an enthusiastic letter reporting the new determination of the velocity of 

light and commending Sperry for the performance 

of his revolving mirrors. The uncertainty in the 

velocity of light has been reduced, he wrote, "from 

something like 100 kilometers to one or two." "Allow me to express my gratification," he continued, 

"at the results given by your steel mirrors. Notwithstanding the great difficulty in figuring [precisely finishing] these to the necessary degree of 

accuracy, the results, as you see by the table, were 

given the highest weight." *^ Sperry, enthusiastically, replied: 

You don't know what great joy you have brought to the 

house of Sperry, I mean the official house as well, by your 

wonderful letter of September 22nd. To think that we have 

been in any way helpful in this great work fairly takes our 

breath away, especially to find that the steel mirrors have 

been operating with high precision, absolutely in the middle 

of the loaded average. That is certainly wonderful.^ 

Analysis and Evaluation of the Dat a 

How Michelson assigned weights to obtain loaded 

averages is not easily determined. The weight assigned a determination made with a particular mirror was not a direct function of the number of observations made with that mirror, as Tables 1 and 2 

clearly show. In an article subsequently published 

in The Astrophysical Journal (1927) about the 

series of experiments of 1924-1926, Michelson did 

not reveal his method. A sentence in his letter to 

Sperry of 22 September 1926, however, suggests that 

Michelson's estimate of precision of his instrumentation was a factor in his evaluation. He judged the 

performance of the instruments, especially the mirrors, with the keen eye of experience and then assigned weights. In the September letter, he disclosed 

that he gave the steel mirrors the highest weights 

despite the great difficulty in figuring them; a clear 

indication that other characteristics of the mirrors 

offset this physical flaw. Another indication that his 

evaluation of instrumentation affected his weighting 

is in Michelson's article of 1924 discussing his preliminary results where he wrote: 

The principal source of error was found to be in the 

maintenance of a sufficiently constant speed of the revolving 

mirror. This was doubtless due to lack of proper provision 

for a constant pressure of the air blast and not to any 

lack of precision in the measurements of the displacement 

of the image. This difficulty will be eliminated in the work 

for next summer, when, it is hoped, the uncertainty of the 

result will be reduced to one in one hundred thousand." 

His drive to improve instrumentation, to weigh 

determinations more heavily, and thereby, hopefully, the precision of his value for the velocity of 

light is also shown by his introduction, in July 

1925, of a vacuum-tube circuit to drive the electric 

fork, which was used to time the revolution of his 

mirrors. (Earlier he had used a make-and-break conTABLE 1.—Michelson's results from observations made in 

1925-1926 (from A. A. Michelson, "Measurement of the 

Velocity of Light between Mount Wilson and Mount San 

Antonio," The Astrophysical Journal, volume 65 (1927), page 

12) 

Mirror 

Glass 8 . . 

Glass 8 . . 

Glass 8 .. 

Steel 8 .. . 

Glass 12 . 

Steel 12 . . 

Glass 16 . 

Glass 16 . 

Weighted 

mean . 

Year N n (km/sec) 

Weight 

1925 

1925 

1926 

1926 

1926 

1926 

1926 

1926 

528 

528 

528 

528 

352 

352 

264 

264 

150 

200 

216 

195 

270 

218 

270 

234 

299,802 

299,756 

299,813 

299,795 

299,796 

299,796 

299,803 

299,789 

299,796±4 

When grouped in series of observations with the five 

mirrors the results show a much more strik-ing agreement, 

as follows. Glass 8: 299,797; Steel 8: 299,795; Glass 12: 

299,796; Steel 12: 299,796; Glass 16: 299,796. 

N=times per second; n=rnumber of observations; V-=2 

velocity of light in vacuo. 

TABLE 2.—Michelson's results from observations made in 

1925-1926 (from Michelson to Sperry, 22 September l926) 

Mirror 

Glass oct 

Steel oct 

Glass 12 

Steel 12 

Glass 16 

Weighted mean ... . 

N 

528 

528 

352 

352 

264 

n 

576 

195 

270 

218 

504 

V 

(km/sec) 

299,799 

299,800 

299,797 

299,800 

299,802 

299,800±1 

Weight 

2 

5 

3 

5 

5 

N=:times per second; n=numbe r of observations; F = 

velocity of light in vacuo. 
NUMBER 37 13 

tact between platinum points to drive the electric 

fork.) The new rate was far more nearly constant,^" but this improvement, for reasons unexplained, is not obvious in the weight Michelson 

assigned to the new series (compare the first and 

second series in Table 1). After the apparatus at 

the home station was slightly rearranged, however, 

and an increase "in intensity in consequence of 

greater effective width of the light beam falling on 

the revolving mirror at nearly normal incidence, as 

well as greater symmetry" ^^ resulted, the subsequent series of observations with the same octagonal 

mirror was weighted three, instead of one, as formerly. 

Additional evidence of the way in which a judgment about the precision of the apparatus probably 

influenced the significance attached to—or the 

weight assigned to—experimental results is found 

in an early Encyclopaedia Britannica article on 

"light" written by Simon Newcomb (1835-1909), 

the distinguished American astronomer who joined 

Michelson in making a determination of the speed 

of light in 1880-1882. Newcomb weighted two series 

of observations zero and a third unity because distortion was removed from the revolving mirror by 

eliminating torsional vibration by regrinding the 

pivots of the mirror.^^ The importance attached by 

Newcomb to variations in experimental apparatus 

is clear from his statement published in 1910: "It 

seems remarkable that since these determinations 

[1880-1882] were made, a period during which 

great improvements have become possible in every 

part of the apparatus, no complete redetermination 

of this fundamental physical constant has been carried out." 53 Using a Sperry searchlight and a Sperry 

mirror, Michelson, in 1926, was responding to the 

opportunity. 

In 1927, Michelson published his results in the 

Astrophysical Journal. Curiously, he gave a different 

value, 299,796 km/sec, in vacuo, from the value, 

299,800 km/sec, reported to Sperry in his letter of 

22 September 1926, despite the fact that the values 

were calculated from the same observations (compare Tables 1 and 2). The only difference in figures 

for the number of observations (n) in the published article and in the letter was that the total for 

the octagonal glass mirror in the published table 

adds to 566, while the number given to Sperry was 

576—possibly a notational error or a simple one of 

addition. In the published series of observations, 

grouped according to the mirror used, each one 

differed from the value supplied Sperry: for instance, the published figure of 299,797 for the 

"Glass 8" differed from that of 299,799 for "glass 

oct." sent Sperry. 

There were also errors and inconsistencies within 

the Michelson Astrophysical Journal article of 1927. 

On page 5 the three sets of observations made with 

the glass octagonal mirrors were reported as having 

been given weights 1, 2, and 5, respectively, while 

in the summary table on page 12 (Table 1), the 

weights assigned were 1, 1, and 3. (If the weights 

1, 2, and 5 are used, then the value of 299,797 is 

obtained, which is the value given in the grouped 

results for the octagonal glass mirror, but is not the 

value, 299,799, that is obtained by using the weights 

(1, 1, 3) assigned in the summary table). The difference is of significance, when it is recalled that 

Michelson told Sperry with great satisfaction that 

the new determinations had reduced the uncertainty to one or two kilometers. Michelson also 

erroneously reported that the first series of observations done with the glass mirror was made in 1925 

(Table 1), while an earlier article in the same 

journal, establishes that the series was run in 1924.^* 

Michelson's chronology was erroneous in another instance: in the 1927 article, he wrote that the large 

octagxDnal glass mirror failed in the summer of 

1925;^° the Sperry correspondence shows the event 

occurred in 1924, after which, in August, Michelson 

wrote to Sperry requesting a steel mirror. Also in 

the earlier article, he published a preliminary determination of the velocity of light that was erroneous because he used an incorrect figure to convert 

the velocity in air to vacuum (the error was corrected in the 1927 article) .^^ 

Another inconsistency is that Michelson's limits 

of uncertainty given in the letter to Sperry and in 

the 1927 article not only differ (±1 as compared 

to ±4) but are not conventional standard deviations, nor weighted standard deviations; they appear to be a rather esoteric standard deviation of 

the mean.3^ Less significant but curious is the use 

in the 1927 Astrophysical Journal article of a full 

page photograph of the first ill-fated steel octagonal 

mirror sent by Sperry, the mirror that would npt 

function because no turbine buckets were provided 

for reversing the mirror. Only an inconspicuous 

footnote explains that the mirror shown was not 

used. 
14 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

This episode in the quest to determine the speed 

of light is punctuated by instances of human mistake and misunderstanding, but the major theme 

to emerge is Michelson's drive to achieve more precision by using improved instrumentation. The 

drive for—and anticipation of—improved instrumentation helps explain Michelson's dogged repetition 

of the "V" experiments, beginning in 1878 and ending only with his death. The Michelson-Sperry correspondence shows concern for near-perfect dynamic 

balance, minimal mechanical deflection of precisely 

machined and highly polished surfaces, and the 

elimination of distortion by the finest figuring. The 

dependence of the scientist upon the instrument 

maker and his tools is obvious. Michelson was 

known as a superbly resourceful designer of experiments; surely his reputation in this regard rested 

in no small measure upon his electing designs that 

exploited ingeniously the state of machine, tool, and 

material technology. In this episode, it was Sperry's 

searchlight and his precision manufacture that provided the technology. 

Michelson's endeavor did not end with the observations of 1926. Despite failing health ( he suffered 

a cerebral hemorrhage at age 77) he designed apparatus for another determination. This time, desirous of avoiding the obstruction of haze and 

smoke and the effects of atmosphere, he used a 

mile-long pipe through which the light beam 

passed in multiple reflections in a near vacuum. 

Throughout 1930 and in 1931 Michelson directed 

the experiments from his sickbed. He died on 9 

May 1931, but his assistants, F. G. Pease and F. 

Pearson, continued the work, finally obtaining a 

value of 299,774 km/sec, in vaciio.^^ 

Sperry had died on 16 June 1930. If he had lived 

on, the exhilaration he knew when he had learned 

that his mirrors were assigned the highest weight 

and operated "absolutely in the middle of the 

loaded average," ^^ would not have been dampened 

by new results with new instruments. Today, the 

value of 299,792.5 km/sec, in vacuo, has been 

adopted by the International Union of Geodesy and 

Geophysics and by the International Scientific 

Radio Union. This varies only 2.5 km/sec from that 

Michelson published in 1927 for the Sperry octagonal steel mirror. It is also noteworthy that Professor R. S. Shankland, who has studied Michelson and 

his work, urges that the determination made by 

Michelson from the 1924—1926 observations be accepted as his best result rather than the later determination made when he was ill, unable to fully 

participate, and completed after he died.^° 
Appendix 

LETTER A 

The University of Chicago 

Ryerson Physical Laboratory 

Aug. 23, 1924. 

Dear Mr. Sperry:— 

Thanks for your prompt reply to my letter. I proceed to 

answer your queries. 

1) With the peripheral speed you quote 27500 ft. per 

minute; the rotor which is to make 540 turns per second 

could be 3 inches in diameter giving a face of the octagon of 

about 114 inches. The thickness should be also I14 in. 

2) We have been making the octagon of glass, and 

mounting on the steel axle thus: 

and this was found to work very well—with an air blast— 

for an octagon one and a half inches diameter but tried with 

a three inch octagon it flew to pieces at about 500 turns. We 

were thinking of trying fused quartz—but I'd be glad to 

have your opinion. (The quartz can be a little more easily 

polished.) 

If it would be possible to make the axle and the octagon 

separately we can make the latter here and. send it on to 

you to be mounted on the axle and balanced. 

3rd. A temperature elevation would not interfere materially with the accuracy. 

4th. Two windows will be required in the case (if a 

vacuum is used) and these we will furnish. 

5th The axle of the mirror is to be vertical— No special 

treatment of frame which is to rest clamped to the iron 

table supporting the whole arrangement. (Probably foot 

screws might be a convenience) 

6th The motor may be below the octagon—we placed 

the paddlewheel for air blast above. 

7th It will be necessary to provide for both positive and 

negative rotations which may require motors above and 

below? 

Trusting for a speedy reply and an estimate of cost—and 

with kind regard 

Very sincerely yours 

A. A. Michelson 

LETTER B 

Mount Wilson 

California 

July 31, 1925 

My dear Mr. SperryIn more ways than one this is the high point of my trip. 

This is the third wonderful day up here on Mount Wilson 

& I doubt if I ever enjoyed three days as much as these. 

In addition to my own profit & enjoyment, however, I 

think I have been of considerable assistance to the folks here. 

They had not been getting the best out of the lamp & had 

even been feeding it by hand, as well as burning with very 

shallow crater. I overhauled the lamp & readjusted it the 

first day. Last evening we had a very satisfactory run & the 

arc behaved like its old self. The first time it has operated 

automatically since they have had it. There is no doubt that 

their light troubles are solved for we could actually see with 

our naked eye the return beam from "Baldy" 25 miles away. 

Not so remarkable sounding until you realize that that 

distant gleam is actually coming from the light that is 

squeezed thru a slit 0.2 mm wide. I wouldn't have believed 

it possible if I hadn't actually seen it. 

This afternoon I have been giving Mr. Pearson & Mr. 

Dowd some instructions on the operation of the lamp. Tonight we will have another test & if all goes well, I will go 

down the Mountain tomorrow morning & look up Selover 

at our Wilmington office. 

Dr. Michelson has not yet been able to get up the mountain & his assistant Mr. Pearson is carrying on the work. They 

have not yet tried out our spinning mirror as they are still 

using their small one until they can perfect their timing 

devices. The main problem now is to know accurately the 

speed of the mirror & they have a room full of clocks, 

tuning forks & other gear in their attempts to get accurate 

timing. It is a tremendous undertaking as there are so many 

factors to be accurately controlled. There is a troublesome 

haze that rolls in & shortens the working time very frequently, 

even though it remains clear overhead. Last evening we had 

to stop work about 9:30 PM on this account, but the big 

lOO" telescope was tending to business as usual & I sat up 

15 
16 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

on the platform with Dr. Joy, the astronomer of the evening, 

& helped him keep his lone watch most of the rest of the 

night. He was taking spectrum photographs of lOth magnitude stars, stars not even visible in a small telescope. 

Dr. Dayton C. Miller is also here and inquired about you. 

He is working on a very interesting experiment to confirm 

(or otherwise) Einstein's Theory. 

I can't begin to tell of the interesting things going on on 

the top of this single Mountain. Even my three days is too 

short to learn of them all. 

I have reported my outbound progress in various letters 

to Mr. Morgan & Mr. Mahoney. I learned considerable about 

the trend of development in the night air mail and have 

enjoyed discussing floodlighting etc with the Westinghouse 

folks in the different cities on the way out. We can undoubtedly cooperate with them to good advantage, as at 

present they have to turn over most big illuminating projects 

to G.E. without even a fight. 

After a day in Hollywood & the Studios I am going to 

San Francisco where I will spend a day or possibly two if I 

can make connections with the Coast Artillery Antiaircraft 

folks. 

Then one day in Seattle, and the Canadian Pacific home. 

I cannot thank Mr. Doran and yourself enough for making 

this trip possible for me. It is being of utmost value to me 

and I hope it will also prove not without values to the 

Company. 

Very truly yours 

Preston R. Bassett 

LETTER D 

October l4. 1925. 

LETTER C 

Aug 22 1925 

Dear Mr. Sperry:— 

Much to my chagrin and owing to an oversight which I 

trust your generosity will attribute to my illness—I find it 

necessary to return to you again the revolving mirror for 

alteration which will allow for rotation in the negative as 

well as the positive direction. Hope this may not be as 

serious a matter as it looks to me—but I am willing to meet 

whatever extra cost this may involve. 

(Mr. Pearson reported that the mirror was O.K. but asked 

me to verify this—which I neglected to do—whence the 

trouble.) 

I'm up on the Mountain and almost restored to health— 

and the work (with our small mirror) is going on very well 

We have already a pretty good result with the original 

set-up and are now in the midst of a series of measurements 

with a modification which looks very promising. 

Nothing seems likely to prevent a determination of "V" 

which is likely to be accurate to within four or five miles— 

except the effect of haze due to forest fires—we are just now 

experiencing one of rather serious character. 

With kind regard 

Sincerely yours 

Mr. Fred Pearson, 

The University of Chicago, 

Ryerson Physical Laboratory, 

Chicago, Illinois. 

My dear Mr. Pearson: 

We are writing this letter to you, preferring not to bother 

the dear old Doctor about little items that are perfectly 

clear in your mind, but not quite clear to us. 

In order to convert the present eight-faced revolving mirror 

into one that may be rotated in either direction, it is 

necessary that a new revolving element be made in order 

to obtain space for two sets of turbine buckets; at the same 

time the web portion of the mirror will be made heavier so 

as to decrease the face deflection at high speeds. This will 

make a slight change in the upper oil chamber casting that 

is easily taken care of. A new casting, between the upper 

and lower casing, is also necessary to provide for the double 

set of nozzles and double air entrance. The distance across 

flats of the mirror will remain the same as in the present 

mirror (3") as well also the vertical face (Ii^"). 

Is it your intention to operate the eight and twelve-faced 

mirrors simultaneously, or one at a time? 

If operated singly, a second rotating element can be provided to go in the same case having a mirror of twelve 

faces, the distance across flats being the same as for the 

eight-faced mirror (3") and the vertical height of the face 

114" as before. Is there any objection to making the vertical 

face measurement, say ^/^", instead of I14", for the twelvefaced mirror so that the faces become nearly square, as in 

the present eight-faced mirror? 

If the two mirrors are to be used simultaneously, this will 

necessitate a complete new machine for the twelve-faced 

mirror. 

In the present machine there are two windows for the 

mirror l80° apart. If a new machine is built for the twelvefaced mirror, do you %vant the two openings l80° apart 

as before, or with 90° spacing? 

If we convert the present machine and provide a second 

mirror, we can have it ready in about five weeks. If we 

convert the present machine and make up a second machine, 

it will require about eight weeks. 

Please drop us a line as soon as possible and set us right 

on just which proposition you have in mind. 

Sincerely yours, 

[unsigned] 

EAS.S. 

A. A. Michelson 
Notes 

1. Sperry to Michelson, 30 September 1926. This letter and 

the Sperry correspondence hereafter cited are from the 

Sperry Papers at the University of Pennsylvania, Philadelphia, Pennsylvania. 

2. On Sperry, see THOMAS PARKE HUGHES, Elmer Ambrose 

Sperry: Inventor and Engineer (Baltimore, Maryland: 

Johns Hopkins Press, 1971) . 

3. NoBELSTiFTELSEN, STOCKHOLM, Les Prix Nobcl en 1907 

(Stockholm: Imprimerie, 1909) , page 8. On Michelson's 

several determinations, see BERNARD JAFFE, Michelson 

and the Speed of Light (Garden City, New York: 

Anchor, i960), and items in the bibliography of Michelson publications appended to ROBERT A. MILLIKAN, 

"Biographical Memoir of Albert Abraham Michelson, 

l852-l93l," National Academy of Sciences Biographical 

Memoirs (Washington, D.C: National Academy of Sciences, 1938) , volume 19. On Michelson, see D. THEODORE 

MCALLISTER, Albert Abraham Michelson: The Man Who 

Taught a World to Measure, Publication No. 3 of the 

Michelson Museum, Naval Weapons Center (China 

Lake, California: Technical Information Department, 

Naval Weapons Center, 1970), an imaginatively illustrated biographical booklet. Also on Michelson, see 

DOROTHY MICHELSON LIVINGSTON (his daughter), The 

Master of Light: A Biography of Albert A. Michelson 

(New York: Charles Scribner's Sons, 1973) . In addition, 

see GERALD HOLTON, "Einstein, Michelson, and the 

'Crucial' Experiment," Isis, volume 60 (1969) , especially 

pages 147-148. Simon Newcomb-Michelson correspondence is in NATHAN REINGOLD, Science in NineteenthCentury America (New York: Hill and Wang, 1964), 

pages 275-306. 

4. Gerald Holton in the foreword to LOYD S. SWENSON, JR., 

The Etherial Aether: A History of the Michelson-MorleyMiller Aether-Drift Experiments, 1880-1930 (Austin, 

Texas: University of Texas Press, 1972), page xix. Swenson's study provides an interesting scholarly account of 

Michelson's experimental testing of the ether theory, 

for which he is perhaps better known than for his 

velocity of light determinations. Swenson discusses the 

dependence of Michelson and other scientists upon the 

greatly improved instruments made possible by the machines and tools of the Industrial Revolution in the 

nineteenth century and after, pp. 58-61. 

I have not discussed the work of Thomas J. O'Donnell, Michelson's regular expert instrument-maker here 

because of the absence of information on him and his 

contribution in the Sperry-Michelson correspondence at 

the University of Pennsylvania and other sources available to the author. The importance of his contribution 

at Mt. Wilson, however, is indicated by R. S. Shankland 

who believes that his participation in these experiments 

helps explain the excellence of the results. R. S. SHANK10. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

LAND, "Final Velocity-of-Light Measurements of Michelson," American Journal of Physics, volume 35 (1967), 

pages 1095-1096. 

MCALLISTER, Michelson, p. 19 [note 3], quoting Michelson from "Recent Progress on Spectroscopic Methods," 

Science, volume 35 (l9ll) , page 901. 

HUGHES, Elmer Sperry, page 291 [note 2]. 

"Gyroscopic Compass," issued September l7, I9l8 (Patent 

No. 1,279,471). 

JAFFE, Michelson, pages 32-33, 5l [note 3]. 

Foucault's use of a revolving mirror had been suggested 

to him by the employment of one by Charles Wheatstone (1802-1875) , the British physicist and telegraph 

engineer, to measure the propagation of electric current 

in a wire. "Jean Bernard Leon Foucault," obituary in 

Proceedirigs of the Royal Society (London, 1869) , volume 

l7, page Ixxxiii. Determinations of the speed of light 

have a long history involving, among others, Ole Roemer 

(1644-I7l0) , and Galileo Galilei. See I. BERNARD COHEN, 

Roemer and the First Determination of the Velocity of 

Light (Norwalk, Connecticut: Burndy Library, 1942). 

The history of the Sperry high-in tensity lamp is told in 

HUGHES, Sperry, pages 215-223 [note 2]. See also Appendix, letter B. 

F. G. Pease to Sperry, 14 August 1924. 

For more on the apparatus, see A. A. MICHELSON, "Preliminary Experiments on the Velocity of Light," The 

Astrophysical Journal, volume 60 (1924) , pages 258-259; 

A. A. MICHELSON, assisted by F. PEARSON, "Measurement 

of the Velocity of Light between Mount Wilson and 

Mount San Antonio,'' The Astrophysical Journal, volume 

65 (1927) , passim; and JAFFE, Michelson and the Speed 

of Light, pages 161-164 [note 3]. 

MILLIKAN, "Biographical Memoir," page l3l [note 3]. 

SIMON NEWCOMB, "Light: Velocity," The Encyclopaedia 

Britannica, 11th edition (New York, l9ll), pages 623-

625. 

A. A. MICHELSON, "Preliminary Measurement on the 

Velocity of Light," Science, volume 60 (3l October 

1924) , page 391. 

MICHELSON and PEARSON, Astrophysical Journal, page 5 

[note 12]. 

Preston Bassett, the Sperry engineer who visited Mt. 

Wilson to help set up the Sperry arc light, reported the 

slit as 0.2 mm wide. Preston Bassett to Elmer Sperry, 31 

July 1925. 

MICHELSON and PEARSON, Astrophysical Journal, page 3 

[note 12]. 

MICHELSON, Astrophysical Journal, page 256 [note 12]. 

Ibid., page 257. 

Michelson to Sperry, 23 August 1924 [Appendix, letter 

A]. 

17 
18 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

22. Sperry to Michelson, 28 August 1924. 

23. Preston Bassett to author, 2l May l97l. 

24. Sperry to H. Nagaoka, 9 March 1925. 

25. Sperry to Michelson, 17 April 1925. 

26. Michelson to A. Schein, Sperry Gyroscope Company, 5 

May 1925. 

27. Michelson to Sperry, 27 April 1925. 

28. Alexander Schein (? no signature) to Michelson, 30 

April 1925. 

29. Sperry (? no signature) to F. Pearson, 7 May 1925. 

30. Ibid.; Sperry to Michelson, 7 May 1925; and Sperry to 

F. Pearson, 8 May 1925. 

31. A three-page handwritten letter dated 23 August l924, 

from Michelson to Speny served as specifications for the 

mirror [Appendix, letter A]. 

32. Michelson to A. Schein, 5 May 1925. 

33. Sperry to Michelson, 7 May 1925. 

34. Sperry (? no signature) to F. Pearson, 7 May 1925. 

35. Sperry to Michelson, 27 May l925. 

36. Ibid. 

37. Sperry to Michelson, 10 June 1925. 

38. Michelson to Sperry, 22 August 1925. 

39. Appendix, letter A. 

40. Sperry to Michelson, 27 August 1925. 

41. Bassett to Sperry, 31 July 1925. 

42. Sperry to Michelson, 23 January 1926. 

43. Sperry to Michelson, 20 October l925. 

44. Telegrams: Michelson to Sperry, 22 October 1925, and 

Sperry to Michelson, 23 October 1925. 

45. Sperry to Fred Pearson, 27 January 1926. 

46. Quote from Michelson letter in J. Spencer Dickenson, 

secretary of the Board of Trustees, to Elmer Sperry, 19 

February 1926. The 12-sided nickel-steel Sperry mirror 

is in the collection of the Michelson Museum at the 

Naval Weapons Center, China Lake, California; the 8-

sided steel mirror with a single row of turbine buckets 

is on exhibit in Michelson Hall at the U. S. Naval 

Academy; and the 8-sided steel Sperry mirror with the 

double row of buckets is on display at the Museum of 

History and Technology, Smithsonian Institution. 

47. Michelson to Sperry, 22 September 1926. 

48. Sperry to Michelson, 30 September 1926. 

49. MICHELSON, Astrophysical Journal, page 261 [note 12]. 

50. MICHELSON and PEARSON, Astrophysical Journal, page 2 

[note 12]. 

51. MICHELSON and PEARSON, Astrophysical Journal, page 5 

[note 12]. 

52. NEWCOMB, "Light: Velocity," page 625 [note 14]. 

53. Ibid. 

54. MICHELSON, Astrophysical Journal, page 260 [note 12] 

gives V (in vacuo) as 299,820 km/sec. Later, MICHELSON 

and PEARSON, Astrophysical Journal, page 2, correct the 

number to 299,802 (Table 1) , because an erroneous correction to vacuum was used in the previous article. The 

299,802, actually obtained in 1924, then, is given as the 

1925 figure in the later Astrophysical Journal article. 

55. MICHELSON and PEARSON, Astrophysical Journal, page 6. 

56. See note 54. 

57. MICHELSON and PEARSON, Astrophysical Journal, page 12 

[note 12], gave as his weighted mean, 299,796 ± 4 (Table 

1). Dean Leon Cooper of the Institute of Technology, 

Southern Methodist University, Dallas, Texas, analyzed 

for me the table of values from which Michelson derived 

this weighted mean: the Michelson calculations were not 

straightforward. The ±4 is not a standard deviation; 

this would be ±l7 . Furthermore, a weighted standard 

deviation would be ±10.4. Michelson seems, therefore, 

to have calculated a "standard deviation of the mean," 

i.e., an estimate of the accuracy of the mean. This 

amounts to ±3.7 and may be the ±4 . In the table in 

the Sperry letter of 22 September 1926, a weighted 

mean of 299,800 ± 1 is given. The ± 1 also can be obtained by calculating the "standard deviation of the 

mean." If the ± 4 and ±1 are standard deviations of the 

mean, Cooper believes some qualification relating to 

confidence intervals would be a more desirable approach. 

For example, values of ± 8 and ± 2 would indicate, respectively, a 95 percent confidence limit for the mean. 

To calculate the standard deviation: 

i=\ 

V2 

N-1 

To calculate a weighted standard deviation, multiply 

(xi—x„,)- for each set of observations by Michelson's 

weight for the set given in the table and use the sum 

of the weights instead of N—1. 

To calculate a standard deviation of the mean, use 

let .X (=:/"' measurement 

X,,,—mean value 

N^^number of observations. 

In this essay, the discussion of Michelson's statistical 

technique has been limited largely to questions raised 

by the Sperry correspondence. For an exhaustive analysis 

of Michelson's technique and the nature of systematic 

errors in the experimental system of 1924—1926, see 

N. ERNEST DORSEY, "The Velocity of Light," Transactions 

of the American Philosophical Society, volume 34 (1944), 

pages 1-110 passim. See also, R. T. BIRGE, "A Survey of 

the Systematic Evaluation of the Universal Physical 

Constants," Nuovo Cimento, supplement to volume 6, 

series lO (1957) , 39-67 passim. 

58. JAFFE, Michelson and the Speed of Light, 165-170; A. A. 

MICHELSON, F. G. PEASE, and F. PEARSON, "Measurement 

of the Velocity of Light in a Partial Vacuum," The 

Astrophysical Journal, LXXXII (1935), pages 26-61. 

59. Sperry to Michelson. 30 September 1926. 

60. ROBERT S. SHANKLAND, "Final Velocity-ot-Light Measurements of Michelson," American Journal of Physics, volume 35 (1967), pages 1095-1096. 

i^ U.S. GOVERNMENT PRINTING OFFICE: 1976 O 599-97 9