﻿of the Sky, and on the Polarization of Light. 387 



This cloud began blue, but the advance to whiteness was so rapid 

 as almost to justify the application of the term instantaneous. The 

 dense cloud, looked at perpendicularly to its axis, showed scarcely 

 any signs of polarization ; looked at obliquely the polarization was 

 strong. 



The experimental tube being again cleansed and exhausted, the 

 mixed air and nitrite-of-butyl vapour was permitted to enter it until 

 the associated mercury column was depressed -fa of an inch. In 

 other words, the air and vapour, united, exercised a pressure not ex- 

 ceeding-^^ of an atmosphere. Air passed through a solution of hy- 

 drochloric acid was then added till the mercury column was depressed 

 three inches. The condensed beam of the electric light passed for 

 some time in darkness through this mixture. There was absolutely 

 nothing within the tube competent to scatter the light. Soon, 

 however, a superbly blue cloud was formed along the track of the 

 beam, and it continued blue sufficiently long to permit of its thorough 

 examination. The light discharged from the cloud at right angles 

 to its own length was perfectly polarized. By degrees the cloud be- 

 came of a whitish blue, and for a time the selenite colours obtained by 

 looking at it normally were exceedingly brilliant. The direction of 

 maximum polarization was distinctly at right angles to the illumina- 

 ting beam. This continued to be the case as long as the cloud 

 maintained a decided blue colour, and even for some time after the 

 pure blue had changed to whitish blue. But as the light continued 

 to act the cloud became coarser and whiter, particularly at its centre, 

 where it at length ceased to discharge polarized light in the direction 

 of the perpendicular, while it continued to do so at both its ends. 



But the cloud which had thus ceased to polarize the light emitted 

 normally, showed vivid selenite colours when looked at obliquely. 

 The direction of maximum polarization changed with the texture of 

 the cloud. This point shall receive further illustration subsequently. 



A blue, equally rich and more durable, was obtained by employ- 

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

 Now the instance 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 tine blue cloud polarizes perfectly the beam 

 which illuminates it, the direction of polarization 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 particles 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 ces- 

 sation of perfect polarization, the residual light which passes when 

 the Nicol is in its position of minimum transmission is of a gorgeous 



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