120 FKAGMENTS OF SCIENCE. 



by Sir Charles Wheatstone to an important observation 

 communicated to the Paris Academy in 1860 by Pro- 

 fessor Govi, of Turin,* M. Govi had been led to ex- 

 amine a beam of light sent through a room in which 

 were successively diffused the smoke of incense, and to- 

 bacco-smoke. His first brief communication stated the 

 fact of polarisation by such smoke; but in his second 

 communication he announced the discovery of a neu- 

 tral point in the beam, at the opposite sides of which 

 the light was polarised 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 maxi- 

 mum polarisation 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 am not 

 aware that M. Govi or any other investigator has pur- 

 sued it further. 



I had noticed, as before stated, that as the clouds 

 formed in the experimental tube became denser, the 

 polarisation of the light discharged at right angles to 

 the beam became weaker, the direction of maximum 

 polarisation becoming oblique to the beam. Experi- 

 ments on the fumes of chloride of ammonium gave me 

 also reason to suspect that the position of the neutral 

 point was not constant, but that it varied with the 

 density of the illuminated fumes. 



The examination of these qtiestions led to the fol- 

 lowing new and remarkable results: The laboratory be- 

 ing well filled with the fumes of incense, and sufficient 

 time being allowed for their uniform diffusion, the 

 electric beam was sent through the smoke. From the 

 track of the beam polarised light was discharged; but 

 the direction of maximum polarisation, instead of being 

 * ' Comptes Rendus,' tome li. pp. 360 and 669. 



