Optische Eigenschaften von natürlichen und künstlichen Bleichromateinkristallen

Out of solution artificial PbCrO4-single-crystals, monoclin modification, equal to natural Crocoit have been produced. The optical constants for natural and these artificial crystals have been found to be equal by measurements of reflectivity and transmission with unpolarised light in the range 0.3 — 15 µm perpendicular to the (110) -plain. Between 1 and 8 µm the refractive index is constant (n = 2,1). In this region some absorption bands occur (2.8 µm corresponding to the donator activation energy of 0.44 eV and 6 μm). At 0.45 µm there is another absorption band but no absorption edge, which was expected with respect to the photoconductivity. Above 8 µm the optical constants are determined by the reststrahlen band at 11.3 μm. The absorption of free charge carriers is discussed because leadchromate in the literature is known to be a purely electronical conductor. An influence of the free charge carriers in the tested spectral range could not be recognized. From this fact it is possible to give some estimations about the conductivity mechanism respectively the mobility of the free electrons.

On the passage of waves through fine slits in thin opaque screens

In a former paper I gave solutions applicable to the passage of light through very narrow slits in infinitely thin perfectly opaque screens, for the two principal cases where the polarisation is either parallel or perpendicular to the length of the slit. It appeared that if the width (2 b ) of the slit is very small in comparison with the wave-length ( λ ) there is a much more free passage when the electric vector is perpendicular to the slit than when it is parallel to the slit, so that unpolarised light incident upon the screen will, after passage, appear polarised in the former manner. This conclusion is in accordance with the observations of Fizeau upon the very narrowest slits. Fizeau found, however, that somewhat wider slits (scratches upon silvered glass) gave the opposite polarisation; and I have long wished to extend the calculations to slits of width comparable with λ . The subject has also a practical interest in connection with observations upon the Zeeman effect. The analysis appropriate to problems of this sort would appear to be by use of elliptic co-ordinates; but I have not seen my way to a solution on these lines, which would, in any case, be rather complicated. In default of such a solution, I have fallen back upon the approximate methods of my former paper. Apart from the intended application, some of the problems which present themselves have an interest of their own. It will be con­venient to repeat the general argument almost in the words formerly employed.

II. Some experiments on metallic reflection. IV. On the amount of light reflected by metallic surfaces. II

In a paper which Professor Stokes did me the honour of communicating to the Royal Society, and which appeared in the “Proceedings,” vol. 35, p. 26, I gave an account of some experiments I had made on the amount of light reflected by polished metallic surfaces when ordinary unpolarised light was incident upon them. The light of a paraffine lamp fell either directly, or after reflection from the metallic surface, on a photometer, and the readings were made by altering the distance at which another similar lamp had to be placed from the photometer in order to produce an equal illumination.