1869.] 



and on the Polarization of Light. 



231 



explanation of the results will probably give new work to the undulatory 

 theory*. 



Before quitting the question of the reversal of the polarization by cloudy 

 matter, I will make one or two additional observations. Some of the 

 clouds formed in the experiments on the chemical action of light are aston- 

 ishing as to form. The experimental tube is often divided into segments 

 of dense cloud, separated from each other by nodes of finer matter. 

 Looked at normally, as many as four reversals of the plane of polarization 

 have been found in the tube in passing from node to segment, and from 

 segment to node. With the fumes diffused in the laboratory, on the con- 

 trary, there was no change in the polarization along the normal, for here 

 the necessary differences of cloud -texture did not exist. 



Further. By a puff of tobacco smoke or of condensed steam blown into 

 the illuminated beam, the brilliancy of the colours may be greatly augmented. 

 But with different clouds two different effects are produced. For example, 

 let the ring system observed in the common air be brought to its maximum 

 strength, and then let an attenuated cloud of chloride of ammonium be 

 thrown into the beam at the point looked at; the ring system flashes out 

 with augmented brilliancy, and the character of the polarization remains 

 unchanged. This is also the case when phosphorus or sulphur is burned 

 underneath the beam, so as to cause the fine particles of phosphoric acid 

 or of sulphur to rise into the light. With the sulphur-fumes the bril- 

 liancy of the colours is exceedingly intensified ; but in none of these cases 

 is there any change in the character of the polarization. 



But when a puff of aqueous cloud, or of the fumes of hydrochloric 

 acid, hydriodic acid, or nitric acid is thrown into the beam, there is a com- 

 plete reversal of the selenite tints. Each of these clouds twists the plane of 

 polarization 90°. On these and kindred points experiments are still in 

 progressf- 



The idea that the colour of the sky is due to the action of finely 

 divided matter, rendering the atmosphere a turbid medium, through which 

 we look at the darkness of space, dates as far back as Leonardo da Vinci. 

 Newton conceived the colour to be due to exceedingly small water particles 

 acting as thin plates. Goethe's experiments in connexion with this sub- 

 ject are well known and exceedingly instructive. One very striking observa- 

 tion of Goethe's referred to what is technically called "chill " by painters, 

 which is due no doubt to extremely fine varnish particles interposed be- 

 tween.the eye and a dark background. Clausius, in two very able memoirs, 



* Brewster has proved the variability of the position of the neutral point for sky- 

 light with the sun's altitude. Is not the proximate cause of this revealed by the fore- 

 going experiments? 



t Sir John Herschel has suggested to me that this change of the polarization from 

 positive to negative may indicate a change from polarization by reflection to polarization 

 by refraction. This thought repeatedly occurred to me while looking at the effects ; 

 but it will require much following up before it emerges into clearness. 



