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Prof. Tyndall on the Blue Colour of the Sky, [Jan. 14, 



endeavoured to connect the colours of the sky with suspended water-vesicles, 

 and to show that the important observations of Forbes on condensing steam 

 could also be thus accounted for. Bruecke's experiments on precipitated 

 mastic were referred to in my last abstract. Helmholtz has ascribed the 

 blueness of the eyes to the action of suspended particles. In an article 

 written nearly nine years ago by myself, the colours of the peat smoke of 

 the cabins of Killarney* and the colours of the sky were referred to one 

 and the same cause, while a chapter of the " Glaciers of the Alps," 

 published in 1860, is also devoted to this question. Roscoe, in con- 

 nexion with his truly beautiful experiments on the photographic power 

 of sky-light, has also given various instances of the production of colour 

 by suspended particles. In the foregoing experiments the azure was 

 produced in air, and exhibited a depth and purity far surpassing any- 

 thing that I have ever seen in mote-filled liquids. Its polarization, more- 

 over, was perfect. 



In his experiments on fluorescence Professor Stokes had continually to 

 separate the light reflected from the motes suspended in his liquids, the 

 action of which he named " false dispersion," from the fluorescent light of 

 the same liquids, which he ascribed to " true dispersion." In fact it is 

 hardly possible to obtain a liquid without motes, which polarize by re- 

 flection the light falling upon them, truly dispersed light being un- 

 polarized. At p. 530 of his celebrated memoir "On the Change of the 

 Befrangibility of Light," Prof. Stokes adduces some significant facts, 

 and makes some noteworthy remarks, which bear upon our present subject. 

 He notices more particularly a specimen of plate glass which, seen by 

 reflected light, exhibited a blue which was exceedingly like an effect of 

 fluorescence, but which, when properly examined, was found to be an 

 instance of false dispersion. "It often struck me," he writes, "while 

 engaged in these observations, that when the beam had a continuous 

 appearance, the polarization was more nearly perfect than when it was 

 sparkling, so as to force on the mind the conviction that it arose merely 

 from motesf. Indeed in the former case the polarization has often appeared 

 perfect, or all but perfect. It is possible that this may in some measure 

 have been due to the circumstance, that when a given quantity of light is 

 diminished in a given ratio, the illumination is perceived with more diffi- 

 culty when the light is diffused uniformly, than when it is spread over the 

 same space, but collected into specks. Be this as it may, there was at 

 least no tendency observed towards polarization in a plane perpendicular 



* 1 have sometimes quenched almost completely, by a Nicol, the light discharged 

 normally from burning leaves in Hyde Park. The blue smoke from the ignited end of a 

 cigar polarizes also, but not perfectly. 



f The azure maybe produced in the midst of afield of motes. By turning the Nicol, 

 the interstitial blue may be completely quenched, the shining, and apparently unaffected 

 motes, remaining masters of the field. A blue cloud, moreover, may be precipitated in 

 the midst of the azure. An aqueous cloud thus precipitated reverses the polarization ; 

 but on the melting away of the cloud the azure and its polarization remain behind. 



