﻿392 Royal Society : — Prof. Tyndall on the Blue Colour 



very irritating to my lungs. The direction of maximum polarization 

 enclosed in this case an angle of 1 2°, or thereabouts, with the axis of 

 the beam. Looked at, as in the former instances, from a position near 

 the electric lamp, no neutral point was observed throughout the entire 

 extent of the beam. 



When this beam was looked at normally through the selenite and 

 Nicol, the ring system, though not brilliant, was distinct. Keeping 

 the eye upon the plate of selenite and the line of vision normal, the 

 windows were opened, the blinds remaining undrawn. The resinous 

 fumes slowly diminished, and as they did so the ring system became 

 paler ; it finally disappeared. Continuing to look along the per- 

 pendicular, the rings revived, but now the colours were complemen- 

 tary to the former ones. The neutral point had passed me in its 

 motion down the beam consequent upon the attenuation of the fumes 

 of resin. 



With the fumes of chloride of ammonium substantially the same 

 results were obtained as those just described. Sufficient, I think, 

 has been here stated to illustrate the variability of the position of the 

 neutral point. The 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 astonishing 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 rever- 

 sals of the plane of polarization have been found in the tube in pass- 

 ing from node to segment, and from segment to node. With the 

 fumes diffused in the laboratory, on the contrary, there was no change 

 in the polarization along the normal ; for here the necessary differ- 

 ences 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 greatlv 

 augmented. But with different clouds two different effects are pro- 

 duced. 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 bril- 

 liancy, and the character of the polarization remains unchanged. 

 This is also the case when phosphorus or sulphur is burned under- 

 neath 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 

 brilliancy 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 hydro- 

 chloric acid, hydriodic acid, or nitric acid is thrown into the beam, 

 there is a complete reversal of the selenite tints. Each of these clouds 



* Brewster has proved the variability of the position of the neutral point 

 for skylight with the sun's altitude. Is not the proximate cause of this revealed 

 by the foregoing experiments? 



