Delisle and Cielo, in a paper appeared in Canadian Journal of Physics 53
, 1047 (1975) (and 54, 2322 (1976)), described an experiment where
two Michelson interferometers were used. The first one was illuminated by
white light and had a large path difference 
(a few metres). The
output was then sent to another Michelson interferometer, whose output was
recorded as a function of his path difference 
.
The output was constant, except when 
, when the
intensity had a strong sinusoidal dependence on 
. This
means that even if the light beam is white (and no spectrometer can resolve
the finely channeled spectrum), the second identical interferometer can act
as a selective detector of this otherwise invisible modification of the
spectrum. They suggested then that this could be used for transmission of
information.
