Fig. 10

Roentgen was aware of the existence of optical birefringence (a crystal is said to be optically birefringent, or double refracting, if its refractive index depends on the polarization and propagation direction of the light with respect to the crystal orientation; a necessary condition is that the crystal is anisotropic; many such birefringent crystals, e.g. calcite and quartz, have a single axis of symmetry). Roentgen ‘s experiments with prisms to observe refraction of X- rays failed, because, as later was measured, the index of refraction of X-rays is many orders of magnitude smaller than that of visible light. Therefore, the likelihood of observing double refraction in birefringent materials would also have been negligible (even if he had been able to realize the experimental conditions as used for visual light experiments, which he wasn’t). Nevertheless he may have wondered if for X-rays the transmission (and not the refraction) was dependent on the crystal orientation with respect to the beam direction. This would explain why he compared the transmission parallel (//) to the symmetry axis of the birefringent crystals ‘Kalkspath’ and ‘Quarz’ with that in a direction perpendicular (⊥) to it. As we now know, the attenuation (and thus the transmission) of X-rays is determined by the atomic number Z, the density and the thickness of the attenuating material, so here no effect was to be expected either. Manually written legend on the photographic paper: ‘Glas, Aluminium, Quarz ⊥, Quarz //, Kalkspath ⊥, Kalkspath //’ (glass, aluminum, quartz ⊥, quartz //, calcite ⊥, calcite //). Paragraph referral: § 3 and § 9)