io5
single reflection in the large etalon. Thus if the optical system is so
arranged that these two beams can interfere, white-light fringes
will be seen when the larger etalon is exactly four times the length
of the smaller. In practice some method of adjustment of the length
of one etalon is necessary and this can be effected by tilting one
of them, the optical length then altering in proportion to the
cosine of the angle of tilt.
The Fabry-Buisson method was used by Sears and Barrell
in their determination of the metre in terms of the wavelength of
the red line of the cadmium spectrum. Since the maximum possible
length for direct measurement with this source is almost 1/9 metre,
Sears and Barrell used two stages of white-light multiplication
to build up to x m.
Figure 8 is a diagram of their apparatus. The first stage M1-M2
is not measured directly by interferometry but is established by
means of a i-metre quartz end-gauge previously so measured.
Then white light from the source L (collimated by lens C) is arranged
to make multiple reflections between M1 and M2 to combine with
a single reflection from Ms. This is accomplished by means of small
holes cut in M2. Thus the separation of M2-M3 can be foundand
so on. The fringes are formed at by means of a telescope lens T.
Of special interest to surveyors is the method of Vaisala which
uses white-light multiplication up to final distances of about
500-900 metres for the establishment of geodetic base-lines.
The Vaisala method differs somewhat from that of Fabry and
Buisson in that the method of compensation to make small
adjustments to the length of one (or both) white-light paths is to
place inclinable optical flats of glass at P. A second difference is
that of using fully silvered mirrors Mv M2, M3, etc., to determine
the length of each stage and introducing holes in each to allow the
appropriate beam of light to enter and leave after making the
desired number of multiple reflections.
When the metre is redefined in terms of a wavelength standard,
how are we to measure the line standards which form a large
fraction of our basic length standards Figure 9 shows one suitable
Fig. 8. Vaisala interferometer.
