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



tion of the matter thus floating in the laboratory-air consists of organic 

 germs, which are capable of imparting a perceptibly bluish tint to the 

 air. This air showed, though far less vividly, all the effects of po- 

 larization obtained with the incipient clouds. The light discharged 

 laterally from the track of the illuminating beam was polarized, 

 though not perfectly, the direction of maximum polarization being at 

 right angles to the beam. 



The horizontal column of air thus illuminated was 1 8 feet long, 

 and could therefore be looked at very obliquely without any disturb- 

 ance from a solid envelope. At all points of the beam throughout its 

 entire length the light emitted normally was in the same state of po- 

 larization. Keeping the positions of the Nicol and the selenite con- 

 stant, the same colours were observed throughout the entire beam 

 when the line of vision was perpendicular to its length. 



I then placed myself near the end of the beam as it issued from 

 the electric lamp, and, looking through the Nicol and selenite more 

 and more obliquely at the beam, observed the colours fading until 

 they disappeared. Augmenting the obliquity, the colours appeared 

 once more, but they were now complementary to the former ones. 



Hence this beam, like the sky, exhibited its neutral point, at op- 

 posite sides of which the light was polarized in planes at right angles 

 to each other. 



Thinking that the action observed in the laboratory might be 

 caused in some way by the vaporous fumes diffused in its air, I had 

 a battery and an electric lamp carried to a room at the top of the 

 Royal Institution. The track of the beam was seen very finely in the 

 air of this room, a length of 14 or 15 feet being attainable. This 

 beam exhibited all the effects observed with the beam in the labora- 

 tory. Even the uncondensed electric light falling on the floating mat- 

 ter showed, though faintly, the effects of polarization*. 



When the air was so sifted as to entirely remove the visible float- 

 ing matter, it no longer exerted any sensible action upon the light, 

 but behaved like a vacuum. 



I had varied and confirmed in many ways those experiments on 

 neutral points, operating upon the fumes of chloride of ammonium, 

 the smoke of brown paper, and tobacco-smoke, when my attention 

 was drawn by Sir Charles Wheatstone to an important observation 

 communicated to the Paris iicademy in 1860 by Professor Govi, 

 'of Turinf. His observations on the light of comets had led M. 

 Govi to examine a beam of light sent through a room in which was 

 diffused the smoke of incense. He also operated on tobacco-smoke. 

 His first brief communication stated the fact of polarization by such 

 smoke ; but in his second communication he announced the discovery 

 of a neutral point in the beam, at the opposite sides of which the 

 light was polarized in planes at right angles to each other. 



But, unlike my observations on the laboratory-air, and unlike the 

 action of the sky, the direction of maximum polarization in M. 

 Govi's experiment enclosed a very small angle with the axis of the 

 illuminating beam. The question was left in this condition, and I 



* I hope to try Alpine air next summer, 

 t Comptes Rendus, tome li. pp. 360 & 669. 



