Skunk Cabbage

The fact that at this stage the polarization is a maximum, when the angle through which the light is turned exceeds a right angle, is the more worthy of note, as the opposite result would prob ably have been expected. By Brewster's law this angle in the case of regular reflection from a plate is less than a right angle; so that not only is the law of polarization for a very small par tide different from that applicable to a plate, but the first effect of an increase of size is to augment the difference.

Sunset Colours.

The simple theory of the scattering of light by small particles suffices to explain not only the blue of the zenith, but the comparative absence of small wave-lengths from the direct solar rays, and the brilliant orange and red coloration of the setting sun and of the clouds illuminated by his rays. The hyposulphite experiment here again affords an excellent illustra tion. But we must not expect a simple theory to cover all the facts. It is obvious that the aerial particles are illuminated not only by the direct solar rays, but also by light dispersed from other parts of the atmosphere and from the earth's surface. On this and other accounts the coloration of the sky is highly variable. The transition from blue to orange or red at sunset is usually through green, but exceptional conditions may easily disturb the normal state of things. The brilliant sunset effects observed in Europe after the Krakatoa eruption may naturally be attributed to dust of unusual quality or quantity in the upper regions of the atmosphere.

To illustrate further the complications that arise when the particle3 are not infinitely small it may be mentioned that if the solution of "hypo" prepared as above be observed for a longer time it becomes more opaque owing to the growth of the sulphur particles and afterwards becomes more transparent again even though kept well stirred ; and further that in this last stage it transmits blue more than red and consequently scatters red more than blue. This is a complete reversal of the blue-sky effect. (Keen and Porter, Roy. Soc. Proc. A. 1914.) A similar phenomenon had previously been observed by Captain Abney and by W. Ritz. Abney says in connection with certain suspensions of silver bromide in collodion: "In some cases I obtained it in such a state which, when viewed by transmitted light, appeared of a sky-blue colour inclining to green." (Abney, Phil. Trans. Roy. Soc. Pt. II., p. 653, 188o W. Ritz, Comptes Rendus 167, 1906.) This phenomenon is well known to preparers of emulsions for photographic plates.

Related to abnormalities of colour we may expect to find cor responding abnormalities in polarization. Of this nature are the neutral points, where the polarization changes character, ob served by F. J. D. Arago, J. Babinet and Sir D. Brewster for an account of which reference may be made to Mascart, Traite d'Optique• (R. ; A. W. Po.)