1884.] modifying the refraction of plane polarised light. 



171 



from direct radiation, but still it is difficult to see how change 

 of temperature of the whole, which does not alter the refractive 

 index of the glass, should produce the effect. Moreover the 

 change was not always observed as the temperature varied and 

 some other cause must be sought. The effect of varying the 

 amouut of moisture in the air round the glass was then tried. 



Dry air was blown from a bellows through a flask filled with 

 Calcium chloride and then through a second with dry cotton- 

 wool on to the surface and damp air was blown through a bottle 

 and then through a second empty bottle to catch the spray. 



Observations were made by means of a wet and dry bulb 

 thermometer placed near the glass on the normal hygrometric 

 condition of the air and of the variations produced by the draughts 

 of dry and damp air respectively. 



The quantity determined directly is the reading of the analyser 

 circle corresponding to the position of the plane of polarisation 

 of the incident light. As the reading increases the angle between 

 the planes of polarisation of the incident and refracted light is 

 increased and vice versa. 



A large number of observations were made at various angles 

 of incidence with the following results. 



Dry air lowers the reading, i.e. brings the planes of polarisation 

 of the incident and refracted waves more nearly into coincidence. 

 Rubbing the surface with dry wash leather lowers the reading 

 which usually comes back after a time to its normal value. The 

 effect of damp air depends on the previous treatment of the 

 surface. If this has not been carefully cleaned with dry leather 

 shortly before the air is blown the damp air lowers the reading, 

 but immediately after the blowing is stopped, or in some cases 



(1) Condition. 



(2) Eeadings. 



Normal 

 Dry air 

 Normal 

 Dry air 



Normal 



Damp Air. 



Observation in act of blowing . 



„ immediately after. 



,, in act of blowing . 



„ immediately after. 



„ in act of blowing . 



,, immediately after. 



32° 45' 



32° 41' 

 32° 45' 

 32° 41'-30 

 32° 44'-30 



32° 38' 

 32° 48' 

 32° 37' 30" 

 32° 47' 30" 

 37' 



32° 

 32° 



12—2 



