exhibited by Polarized Light. 469 



13. A rotating velocity, which continues to decrease and 

 increase through an entire revolution, in (1) and (9) produces 

 partially polarized instead of ordinary light. 



14. Unannealed glasses rotating between Nichol's prisms at 

 rest, do not neutralize each other and form white light. 



15. Light from a rotating Nichol's prism falling upon a 

 metallic mirror, possesses the ordinary characters, 



16. The twin-growth of crystals gives rise to very compli- 

 cated phaenomena of colour. This is very beautifully seen in 

 plates of calcareous spar, ground at right angles to the axis 

 of the inclosing individual. The author has formed some of 

 them artificially, by introducing a lamina of selenite of a de- 

 finite thickness between two accurately centred plates*. If a 

 plate of a natural twin-crystal be rotated, after having been 

 accurately centred as regards the axis of rotation, the colours 

 become neutralized, forming white light ; whilst a simple plate, 

 rotated on its axis, preserves its system of rings unchanged. 



N.B. [The phaenomena o? interrupted crj/stals, described in paragraph 16 

 of this paper, were observed by Sir David Brewster previous to 1819; and 

 the method of forming them artificially by enclosing laminas of selenite and 

 mica between accurately centred plates of calcareous spar, and between 

 equal plates of the spath calcaire basse, was practised by him previously to 

 March 1827, the date of which is signed, in his Journal of Experiments, by 

 the Vice-President of the Royal Society of Edinburgh. The experiments, 

 too, are briefly described in his Treatise on Optics, Part II. chap. xxii. ^115, 

 p. 197.— Ed.] 



LXXIII. On the Depolarization of Light. By Prof. Dove. 



IF rectilinearly polarized light be allowed to fall upon a 

 rough surface, as a white wall, it is perfectly depolarized. 

 This phaenomenon is most distinctly seen, by allowing the 

 spectra of a prism of rock-crystal, polarized at right angles to 

 each other, to fall upon such a surface, and analysing the 

 purple-red resulting from the violet end overlapping the red 

 extremity, with a Nichol's prism. On rotating the latter, there 

 is not the slightest alteration in colour. This depolarization, 

 which eight years ago the author, in a paper communicated to 

 the Berlin Society of Friends of Natural History, showed also 

 to occur on the rough internal surface of bodies covered with 

 a reflecting layer, progressively diminishes with the obliquity 

 of the incidence, so that even the dullest surfiice ceases to de- 

 polarize at very acute incidences. With perpendicular inci- 

 dence, it also extends to circular and elliptical light, which, 

 when reflected by a rough surface, is depolarized, i.e. possesses 

 the properties of common light. If the explanation of the 

 depolarizing property of rough surfaces is found in the sup- 

 position that such a surface reflects in all directions, by the 

 converse, the combination of light polarized in all possible 

 " PoggendorfTs Annalcn, vol. xxxv. p. 594. 



