cadmium lamp. Adjacent to each photograph is the observer's
estimate (mean of two readings) of the excess fraction a/brelating
to the particular wavelength concerned. It is seen that for the
cadmium red line the length of the gauge happens to be almost
exactly an integral number of half-waves. Figure 6 is a diagram
of the complete instrument. The prism shown selects any particular
spectral line.
In practice it is not necessary to count the orders of interference
for any particular spectral lineif the length of the gauge is known
with adequate approximation and the fringe fractions are measured
for a number of colours, there is only one computed length near to
the approximate one which will give the measured set of fractions.
Many other forms of interferometers have been adapted to meas
uring end-gauges, but I do not think it would be appropriate to
discuss them here.
Is it possible to extend the accuracy of interferometric measure
ment to very long distances Yes it isCuriously enough the means
whereby this extension of range is achieved does not employ
monochromatic light, but instead uses white light.
The principle of white-light multiplication of a small distance
depends upon the fact that interference will only take place when
the path difference between interfering beams is almost zero.
Indeed, the only colourless fringe is obtained for complete equality
of path difference and this is usually accompanied by one or two
rainbow-coloured fringes on either side, rapidly merging into a con
tinuous white-light background.
Figure 7 illustrates the principle of the white-light multiplier
first suggested by Fabry Buisson (1919) (3). The device is
composed of two "etalons" in line, each consisting of a pair of
parallel partially metallised beam dividers. The smaller etalon
is of such a length that it can be measured in monochromatic light
and the larger etalon is arranged to be an almost exact integer
number of times (for example: four times) the length of the small
etalon. It is seen that if a beam of white light is passed through
both etalons the portion which is reflected four times in the small
etalon traverses the same distance as one which experiences a
104
Fig. 7. Fabry and Buisson method of compensation by inclining the larger
optical length as used by Sears and Barrell.
e< -q&ne,
e^ne, e, inclined e? normal to telescope axis
Telescope Axis
(2)
(1)
