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Abstract. -The distribution of the interstellar dust inside the LMC has been investigated on the basis of UBV photometry and spectral classifications of 1507 luminous O-A type stars. The mean internal reddening has been found to be rather high with EB−V = 0. m 16. The frequency distribution of the reddenings is strongly asymmetric, so that it is useless to give an error of the mean. Reddenings up to 0. m 8 have been observed. A strong selection effect is caused by the restricted visibility of strongly reddened stars. The intrinsically faint stars (V0 > 13. m 3) of our sample show no or low reddening EB−V < 0. According to the luminosity function the completeness limit of our sample has been found to be about 12.m . To determine the reddening without bias, all intrinsically fainter stars have to be rejected. A comparison of the positions of highly reddened stars with EB−V > 0.3 with those of molecular clouds detected by Cohen et al. (1988) shows few coincidences. Contrary to the dark cloud catalogues of Hodge (1972) and van den Bergh (1974) we detect highly reddened stars almost everywhere in the LMC. The frequency distribution of the reddenings can be fitted by a two cloud model. The mean reddenings caused by small and large dark clouds are 0. m 04 ± 0. m 01 and 0. m 40 ± 0. m 10, respectively. In the LMC, dust cloud properties are quite similar to those in the Galaxy. The number of small clouds on line of sight in comparison to that of large clouds is, however, higher than in the Milky Way. For the LMC we observe a number ratio of 45 ± 10 which may be compared with the galactic value of 9 ± 1. Finally we have determined a map of the internal reddening in the LMC. It shows overall agreement with that of Isserstedt & Kohl (1984), but gives higher absolute values, dominated by 30 Dor and the supershell LMC 2. Rather high reddenings have been found in the HII region N 11 and around the supershell LMC 6, too. Our map shows the same structure of the interstellar medium in the LMC as the HI map derived by Luks & Rohlfs (1992) and the IR emission maps derived by Israel & Schwering (1986).Key words: galaxies: Magellanic Clouds -interstellar medium: extinction The Data baseThe UBV data and the spectral classifications were taken from the Bochum LMC Data Base (Gochermann et al. 1992). As almost all photometric observations were carried out using the Johnson system these data can be treated as rather homogeneous. The homogeneity of the observations carried out by the authors themselves is discussed in Oestreicher et al. (1995). The spectral classifications based on slit spectra can be treated as homogeneous (see Fitzpatrick 1988;Conti et al. 1986 andMassey et al. 1995). The classifications based on objective prism spectra, however, are rather inhomogeneous as already discussed by Ardeberg et al. (1972), Stock et al. (1976) and Rousseau et al. (1978).2353 stars were available. After omitting all stars with uncertain photometry or spectral type or with spectral pecularities and all double and multiple systems, however, only...

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Checking the density-wave theory against observations of our own Galaxy has proven very difficult, as witnessed also at this Symposium. Less ambiguous results, however, are obtained for other galaxies. These results involve a) calculating convincing models for a sample of 25 fairly well observed spirals (Roberts et al. 1975) and b) locating the compression zones on the inner edges of the spiral arms.

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