fringes formed by multiple reflections of light between a pair of
partly metallised plates, each serving the dual purpose of beam
division and recombination. With the plates parallel (top picture)
and convergent illumination from a broad source, circular fringes
are obtained. With parallel light and inclined plates we obtain
straight fringes.
The use of these straight fringes formed by slightly inclined plates
has found great application to the precision measurement of engi
neer's block gauges ("slip" gauges). These are made in various
lengths up to about 30 cm and are rectangular pieces of steel of
section about 9 mm x 34 mm, the length being defined by the
distance between the opposite optically flat and parallel end faces
of this cross-section.
Figure 4 shows the principle of a gauge-measuring interferometer
developed at the National Physical Laboratory (2). The gauge to be
measured (G) is wrung (i.e. placed in such close contact that it is
held in position by cohesive forces) on to a large optically flat steel
base-plate. The beam divider is a glass or quartz optical flat
partially metallised (60-70% reflectivity) on its underside, placed
above the upper surface of the gauge. The parallel light is obtained
by means of a small aperture and collimating lens; the eyepiece
aperture is arranged to be effectively close to the source aperture,
so the collimating lens also acts as a telescope. The beam reflected
from the partially metallised surface can interfere with the coherent
101
OPTICAL FLAT
GAUGE
G H h
2 h JVjX -f- X
2 H N 2X-f- ^X-f- X
b
2 G (NiVjjX-f- X
b
G (N2-N1)-+^
2 b 2
Fig. 4. Basis of gauge interferometer.
