The apparatus present in the museum is made all of metal painted black and is supported by a vertical column (21.5 cm in height) attached on the side to a heavy base on which are three equidistant brass levelling screws supporting a round platform (diameter 10.8 cm) which is connected to the base at the centre by means of a metal spring that holds it tight to the levelling screws.
On the platform, which is covered with black velvet, a glass plate is placed to produce the interference fringes.
A horizontal arm attached to the end of the column supports the tube of the device (length 13 cm, diameter 4.3 cm) the axis of which passes through the centre of the platform. In the lower part of the tube there is an objective lens that can be moved along the axis by about 1.5 cm. At the top, near the focal plane of the objective lens, there is an iris diaphragm the aperture of which can be adjusted by means of a ring. At the top, the tube ends above the iris diaphragm with a box in the shape of a parallelepiped, in which there are two total reflection prisms.
One of these receives the beam of light from the source, either directly or through a horizontal collimator (length 9.5 cm; diameter 3.2 cm), supported by a shaft about 4 cm in length placed at the top of the column, and reflects it downwards through the tube onto the plate of glass on the platform.
The other prism reflects horizontally, on the side opposite to that of the collimator, the beam of light coming from the plate of glass which has passed through the tube, sending it on to the eye of the observer through the eyepiece or directly when the eyepiece, which is connected to a lever, has been moved up.
A blackened screen which has at its centre a square aperture (2 x 2 cm), placed between the collimator and the tube, protects the observer's eye from the light coming directly from the source. As the light source a mercury arc lamp with a green (446 nm) or blue (436 nm) filter is used, but for the Fizeau fringes it is also possible to use a sodium lamp (589 nm; 589,6 nm).
Hecht - Zajac (1974), pp. 295-299
Landsberg (1979), Vol. I, pp. 118 e 126
Palmer (1962), p.171
Smith (1960), pp. 488-510