Hydrogen atoms in Rydberg states (principal quantum number
n
≫ 1) inserted between parallel mirrors separated by a distance
L
suffer level shifts now becoming measurable. The analysis in the preceding paper is applied to predict these shifts ⊿. When
L
<
n
3
a
/α (
a
is the Bohr radius; α =
e
2
/
ħc
≈ 1/137), ⊿ is basically electrostatic (‘non-retarded’), and of order (
n
/
4
/
L
3
mm
) x 10
‒6
Hz (with
L
expressed in millimetres). When
L
> n
3
a
/α, ⊿ is basically radiative (‘retarded’), and of order (2/
n
2
L
mm
x 10
4
Hz. Fine structure and hyperfine structure are taken into account, considering the extreme cases of low orbital angular momenta (
l
= 0, 1), and, more briefly, very high
l
(with (
n
—
l
)/
n
=
O
(1/
n
)). Explicit formulae are given in the non-retarded régime, except for very small
L
, where ⊿ becomes comparable with the fine structure. In the retarded régime for low
l
, the requisite radial integrals are not available accurately, and only rough predictions can be made.
