part transmitted and subsequently reflected back and forth between
the beam divider and the base-plate, or between the beam divider
and the top surface of the gauge.
This type of interferometer is said to be of the Fizeau type.
When the beam divider is slightly inclined relative to the base-plate
straight fringes are seen, the fringes from the top of the gauge
usually being displaced relative to those from the base-plate.
If G equals the length of the gauge, G N X/2 (a/b) (X/2) (fig. 4)
where N N2 exact number of orders of interference
(i.e. half-wavelengths) in the length of the gauge and afb is termed
the "excess fraction" of an order by which the length of the gauge
exceeds N. X is the wavelength in air of the particular monochro
matic light used. This is derived from the vacuum wavelength
and a knowledge of the refractive index of the air. (See later and
reference 8). a[b is obtained visually from the displacement of the
fringes from the top of the gauge relative to those from the base
plate.
Figure 5 is a photograph showing fringes from this type of inter
ferometer for four different spectral lines emitted by a laboraroty
102
Red Green
Blue Violet
Fig. 5. Gauge interferometer observations.
