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



nitrite-of-butyl vapour in a still more attenuated condition. Now the in- 

 stance here cited is representative. In all cases, and with all substances, 

 the cloud formed at the commencement, when the precipitated particles 

 are sufficiently fine, is blue, and it can be made to display a colour rivalling 

 that of the purest Italian sky. In all cases, moreover, this fine blue cloud 

 polarizes perfectly the beam which illuminates it, the direction of polari- 

 zation enclosing an angle of 90° with the axis of the illuminating beam. 



It is exceedingly interesting to observe both the perfection and the decay 

 of this polarization. For ten or fifteen minutes after its first appearance the 

 light from a vividly illuminated incipient cloud, looked at horizontally, is 

 absolutely quenched by a Nicol's prism with its longer diagonal vertical. 

 But as the sky-blue is gradually rendered impure by the introduction of par- 

 ticles of too large a size, in other words, as real clouds begin to be formed, 

 the polarization begins to deteriorate, a portion of the light passing through 

 the prism in all its positions. It is worthy of note that for some time after 

 the cessation of perfect polarization the residual light which passes, when 

 the Nicol is in its position of minumum transmission, is of a gorgeous blue, 

 the whiter light of the cloud being extinguished*. When the cloud texture 

 has become sufficiently coarse to approximate to that of ordinary clouds, 

 the rotation of the Nicol ceases to have any sensible effect on the quality of 

 the light discharged normally-. 



The perfection of the polarization in a direction perpendicular to the illu- 

 minatingbeam is also illustrated by thefollowing experiment. A Nicol's prism 

 large enough to embrace the entire beam of the electric lamp was placed 

 between the lamp and the experimental tube. A few bubbles of air carried 

 through the liquid nitrite of butyl were introduced into the tube, and they 

 were followed by about 3 inches (measured by the mercurial gauge) of air 

 which had been passed through aqueous hydrochoric acid. Sending the 

 polarized beam through the tube, I placed myself in front of it, my eye being 

 on a level with its axis, my assistant Mr. Cottrell occupying a similar 

 position behind the tube. The short diagonal of the large Nicol was in the 

 first instanee vertical, the plane of vibration of the emergent beam being 

 therefore also vertical. As the light continued to act, a superb blue cloud 

 visible to both my assistant and myself was slowly formed. But this cloud, 

 so deep and rich when looked at from the positions mentioned, utterly 

 disappeared when looked at vertically downwards, or vertically upwards. 

 Reflection from the cloud was not possible in these directions. When the 

 large Nicol was slowly turned round its axis, the eye of the observer being on 

 the level of the beam, and the line of vision perpendicular to it, entire extinc- 

 tion of the light emitted horizontally occurred where the longer diagonal of 

 the large Nicol was vertical. But now a vivid blue cloud was seen when 

 looked at downwards or upwards. This truly fine experiment was first defi- 

 nitely suggested by a remark addressed to me in a letter by Prof. Stokes. 



* This seems to prove that particles too large to polarize the blue, polarize perfectly- 

 light of lower refrangibility. 



