The general theory of holographic fringe contrast in the presence of a
transparent test cell in the object beam is presented. A specific cas
e of triglycine sulphate aqueous solution with a concentration gradien
t in the test cell is also described in detail. The general irrational
ly related as well as frequency-doubled wavelengths for two-colour hol
ography are considered and consequences on holographic fringes and the
ir contrast are discussed.