122 FRAGMENTS OF SCIENCE 



tained a decided blue color, and even for some time after 

 the 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 dis- 

 charge polarized light in the direction of the perpendicular, 

 while it continued to do so at both ends. 



But the cloud which had thus ceased to polarize the 

 light emitted normally, showed vivid selenite colors when 

 looked at obliquely, proving that the direction of maxi- 

 mum 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 employing the nitrite -of -butyl vapor in a still more at- 

 tenuated condition. The instance here cited is represen- 

 tative. In all cases, and with all substances, the cloud 

 formed at the commencement, when the precipitated par- 

 ticles are sufficiently fine, is hlue^ and it can be made to 

 display a color 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 polarization 

 enclosing an angle of 90° with the axis of the illuminating 

 beam. 



It is exceedingly interesting to observe both the perfec- 

 tion and the decay of this polarization. For ten or fifteen 

 minutes after its first appearance the light from a vividly 

 illuminated actinic cloud, looked at perpendicularly, is ab- 

 solutely quenched by a NicoFs prism with its longer diag- 

 onal vertical. But as the sky-blue is gradually rendered 

 impure by the growth of the particles — ^in other words, as 

 real clouds begin to be formed — ^the polarization begins to 

 decay, a portion of the light passing through the prism 

 in all its positions. It is worthy of note, that, for some 



