I 



Proceedings of the British Association. 165 



the edge dividing the ray were directed to the red end of the 

 spectrum, then fringes were seen : but no such fringes appeared 

 when it was turned to the violet end of the spectrum. One pe- 

 culiarity of these fringes, not before noticed, was that they had 

 not the forms of bands, but rather the appearance of screws, or 

 dotted black lines, or as if they were formed by the shadow of a 

 plate of metal perforated by small openings. This, which ap- 

 peared to be a new property of light, and to indicate a polarity 

 in the simple rays of light, when separated from each other by 

 refraction, he had commented on at the meetings of the Associa- 

 tion at Liverpool and Bristol : and Mr. Airy, the Astronomer 

 Royal, had given a paper and two publications on the subject, in 

 which he endeavored to account for this upon the undulatory 

 theory, arguing that the apppearance and magnitude of the fringe 

 depended upon the diameter of the pupil or of the object-glass. 

 Sir D. Brewster said, he had repeated all his experiments under 

 every variety of form, varying the diameter of the pupil from its 

 greatest expansion to its greatest contraction, and the diameter of 

 the object-glass from four inches to a quarter of an inch, and the 

 fringe remained utterly unaffected by these variations. He fur- 

 ther found, that these fringes varied in magnitude with the dis- 

 tance of the eye from the refracting body, and not with the 

 magnitude of the pupil. He stated several other results, all of 

 which, he thought, could not be explained on the principles of 

 the undulatory theory. This paper gave rise to much discussion ; 

 the prevailing opinion appearing to be that future discoveries 

 would probably reconcile the phenomena in question with the 

 wave-theory of light. 



Sir D. Brewster made a communication on the Dichroism of 

 the P alladio-chlorides of Potassium and Ammonium. Dr. Woi- 

 laston had found that a long crystal of either of these salts, when 

 looked through transversely, had a green color, but when looked 

 through from either end, had a red color; and he (Sir David) 

 placed one of these long crystals transversely over another, in a 

 cruciform shape, and then found that those portions of the cen- 

 tres of both, which were in contact, gave a red color, while all 

 the ends of the crystals were red. 



On the Geometric forms } and laws of illumination of the spaces 

 which receive the solar rays, transmitted through quadrangular 

 apertures; by Sir D. Brewster. His attention was called to 



