1884.] modifying the refraction of plane polarised light. 



173 



The angle of incidence here was about 60°, the temperature 

 varied from 23° C. to 24° C. and the tension of aqueous vapour 

 from ll"3mm. to 14*2 mm. the saturating tension changing from 

 19-7 mm. to 217 mm. 



During the first eleven observations the glass had not been 

 cleaned since the previous day, and the effects are the same as 

 previously. After the glass was cleaned the damp air raises the 

 reading without previously lowering it. 



Other sets of observations at different angles of incidence gave 

 the same results. 



Thus on August 20 we find just after the glass has been 

 cleaned at angle of incidence of 45° the following readings. 



There seems too a tendency for the normal readings to rise 

 with e the saturating tension, this is shewn below. 



The connexion however is not very marked. 



It remains then to account, if possible, for the anomaly in the 

 behaviour of the glass with damp air ; but before attempting this 

 it is well to mention that the effects produced by blowing other 

 gases Oxygen, Hydrogen, and Carbonic acid on the surface, were 

 tried and found to be practically inappreciable, while additional 

 experiments made in October, 1884 confirm the results already 

 arrived at as to the effects of dry and damp air. 



Observations also were made on the amount of saturation 

 produced ; damp and dry air being blown on to the wet bulb. 



