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Type A‐centres are found in KJ:Na single crystals. The conversion F → A is followed in the visible region and near the fundamental absorption edge. The intensity of the β‐band decreases with the formation of A‐centres and at the same time a new band (βA) is formed at longer wavelengths. The disappearance of A‐centres results in a broad Ac‐band in the visible region which is accompanied by the formation of an αA‐band in the ultra violet region, at wavelengths shorter than those corresponding to the α‐band. As the intensities of the A1‐and βA‐bands are proportional the occurrence of the βA‐ and αA‐bands can be explained qualitatively by exciton formation near A‐centres or vacancies which are due to negative ions with a Na ion as nearest neighbour.

The effective Madelung energy of a transition leading to the generalization of the law of Hilsch and Pohl is &fined. A very simple empirical method is given for the determination of the effective Madelung energy of a transition in crystals. The method enables to estimate the position of the maxima for bands a, By U(H-), I-, A(Tl+), B(Tl+), C(Tl+), D(Tl+), C(Ag-), etc. Also, valuable indications are obtained regarding the nature of transition and the mode of participation of the lattice ions in the transition. On dbfinit l'bnergie Madelung effective d'une transition qui m h e B la gbnbralisation de la loi Hilsch et Pohl. Une mbthode empirique fort simple pour la dbtermination de l'hergie Madelung effective d'une transition en cristaux est donnbe. Cette mbthode permet l'estimation de la position des maxima pour les bandes a, 8, U(H-), I-, A(Tl+), B(Tl+), C(TI+), D(Tl+), C(Ag-), etc. De m6me, de prbcieuse indications sont obtenues se rapportant B la nature de la transition et au mode de participation des ions du r6seau B la transition.

Um ein Modell für das A‐Zentrum aufzustellen, wurde die optische Absorption in natürlichem und polarisiertem Licht an Li‐dotierten KCl‐Kristallen gemessen. Die Ergebnisse sind in guter Übereinstimmung mit denen anderer Autoren, die Na‐dotierte KCl‐Kristalle untersucht haben, und können mit Hilfe des A‐Zentrenmodells, das aus einem Na‐ oder Li‐Ion in unmittelbarer Nachbarschaft einer Anionenleerstelle besteht, erklärt werden.