24 Mr. R, H. M. Bosanquet on a new Form of Polariscope, 



on the contrary, the action of diminished thickness corresponds 

 to left-handed rotation, and the bands appear to move to the 

 centre. Thus for rotation of analyzer with watch the bands 

 move from the right-handed quartz to the left-handed one, 

 and vice versd. 



In white light, as the breadth of one band is different for 

 each colour, and the rotation of 180° in either the incident 

 polarization or the analyzer displaces every colour through 

 one band, the effect of rotation on the bands is complex ; but 

 they remain substantially equally visible in all positions *. 



Application to the observation of the shy. 



In a paper read before the Ashmolean Society in 1875 f, I 

 pointed out the untenableness of the conception of neutral 

 points as separating the positive and negative regions of po- 

 larization in the sky, and suggested that it was probable for 

 various reasons that these two regions were separated by a 

 ring in which the polarization passed through a value zero. 

 At that time I had not succeeded in observing the polariza- 

 tion in the neighbourhood of the neutral points at all. Great 

 difficulty was found, even with the Savart, in settling the 

 point ; and the polariscope above described was constructed 

 for this purpose. With the assistance of this and the Savart, 

 which supplement each other's indications in a useful manner, 

 I have made out the state of the facts. 



The tract which separates the regions of positive and nega- 

 tive polarization exhibits, elsewhere than in the neutral point, 

 polarization of sensible amount, which is neither positive nor 

 negative, but is in planes inclined at angles other than right 

 angles to the plane through sun, point, and observer. That is 

 to say, the direction of the polarization changes from that 

 plane to a direction at right angles with it by a gradual rota- 

 tion through the right angle, and not by passing through a 

 zero value, unless we cross over the neutral point itself. 



The first fact in support of this statement is, that it has been 

 ascertained beyond a doubt by the new polariscope that the 

 polarization is continuous between the positive and negative 

 regions ; i. e. the bands never vanish while the instrument is 

 slowly passed from a point in the negative region to a point 

 in the positive one, unless we cross over the neutral point 

 itself. 



The property of the Savart then comes in usefully — that it 



* The instrument was constructed by Messrs. Tisley and Spiller. It is 

 now in the Loan Exhibition of Scientific Instruments at South Kensington. 

 t Phil. Mag. Dec. 1875, Suppl. 



