JL
B\
io6
iiMimiiiiiiiiiiii'iiiiiiiiiijiiiiiiiiiii
Fig. 9. Mesure d'un même déplacement sur la règle a traits, placée sous
le microscope, et sur l'épaisseur de la lame d'air entre les miroirs A et B.
method, suggested by Volet Cabrera. The microscope is set
on one fiduciary line on the line-standard which is then moved
until the next fiduciary line is central under the microscope. The
movement can be measured by means of the change in the spacing
of the interferometer plates A, B, one fixed (B), the other attached
to the end of the movable line standard.
The Velocity of Light as a Standard of Length
In this heading I am using the term "light" in rather a loose
sense, for I suppose I really ought to say "electromagnetic waves".
The most widely appreciated application of the velocity of electro
magnetic waves to distance measurement is, I think, that of radar.
The distance of a reflector is deduced from the time it takes for a
short pulse of microwaves to travel to and from it. A knowledge
of the vacuum velocity of the waves is required, together with a
knowledge of the refractive index of the air under the prevailing
atmospheric conditions.
Radar requirements are not in general so precise as those for the
measurement of geodetic bases, and it is the development of these
geodetic high precision distance measuring devices which has re
quired a much more accurate value of velocity than ever before.
I will first discuss some of the recent precision determinations
of the velocity of "light".
At the end of the war the accepted value was that recommended
by Birge (4) (vacuum velocity, C0 299776 4 km/s) from a
statistical review of other workers' results. The first of the post-war
measurements to show the inaccuracy of Birge's value was that
of Essen and Gordon-Smith (5) who used the microwave cavity
resonator method. This was quickly followed by Bergstrand (6)
using his "Geodimeter" which is based on a modulated light-beam
method involving the measurement of velocity over distances of
the order of 10 km, whereas the cavity resonator uses distances
of only about 10 cm.
