A description is given of an optical method for the determination of the thickness of the helium II film. Linearly polarized light is reflected from a stainless steel mirror, the upper part of which is coated with a layer of barium stearate one molecule thick and the lower part with a similar layer three molecules thick. The reflected light passes through a mica compensating plate and a nicol prism. Adjustment of the mica plate and nicol gives equality of illumination on the ‘1’ and ‘3’ areas. If now a film of helium II covers the mirror the nicol must be rotated to restore equality of illumination. The rotation is a measure of the thickness of the helium film, the relation between the two quantities being calculated in terms of the angle of incidence and the optical constants of liquid helium, barium stearate and stainless steel. The observed thickness at any given height above the liquid helium was found to be nearly independent of the temperature between 1⋅1 and 2⋅18° K but then decreased rapidly to zero at the
λ
-point. In the formula
d
=
k
/
h
1/
z
for the thickness
d
at height
h
cm. for any given temperature, the value of
z
varies from 3⋅5 at 1⋅1° K to 2⋅5 at 2⋅1° K. The thickness at 1 cm. and 1⋅5° K is provisionally given as 1⋅9 x 10
-6
cm.
Quantum reflection of antihydrogen from a liquid helium film
