324 Sir David Brewster on the Coloured Hunppes 



degrees for the diameter of both. I have measured them with 

 great care, aud have received measurements from others, and I 

 tiud the greatest diameter of the two to be 4| degrees. 



2. The houppes or coloured sectors have a diiferent appear- 

 ance to different persons. The Abbe Moigno describes them, 

 and M. Haidinger has drawn them, as resembling a bundle of 

 pale-yellow twigs bound tightly together at their middle, and 

 having on each side of the narrowest part of the bundle two 

 small masses of violet or blue light*. Afterwards, however, the 

 Abbe made a most important observation, described and drawn 

 by M. Haidingcrf. He observed the blue masses or sectors cross- 

 ing the middle part of the yellow bundle now represented by this 

 separation, as consisting of two yellow circular spaces. 



In the numerous observations which I have made, the yellow 

 sectors have the appearance as drawn by M. Haidinger, that is, 

 there is a certain breadth of yellow light in the narrow part of 

 the bundle of yellow twigs; but they have this appearance only 

 tvhen they have a vertical position, that is, when they are perpen- 

 dicular to a line joining the eyes. At right angles to this position 

 the blue sectors or masses encroach upon the yellow, and occupy 

 the middle of the yellow bundle. When the head is turned round, 

 the yellow bundle with its middle part turns round also, and is 

 always perpendicular to the line joining the eyes, while the blue 

 masses or sectors united are always in that line. Reckoning 

 I'rom the middle point of the yellow sectors, the angle formed by 

 each of them does not exceed 65 degrees, so that the angle of the 

 blue sectors must be 115 degrees each. 



3. The colour of the houppes or sectors is a very faint yellow, 

 and a pale blue fully as bright as the yellow. 



According to M. Jamin, the yellow sectors are nothing more 

 than portions of the polarized beam which are refracted more 

 copiously by the cornea and crystalline when the refraction is 

 made in or near a plane perpendicular to the plane of primitive 

 polarization, than when it is refracted in or near that plane. 



" The refracted light," he says, " will therefore exhibit in the 

 plane of polarization two obscure sectors {aigrettes) united at the 

 centre by their summits, widening towards the circumference, 

 and tivo brilliant sectors of the same form in the perpendicular 

 plane. 



The colour of the light thus refracted must be slightly yellow, 

 as I have long ago provedj; and M. Jamin finds in this yellow 

 light the cause of the yellow seotors, while he regards the two 



* Re[)crloire d'Optique Moderne, p. 1326. 



t Poggcndorff's Annakn, vol. Ixvii. p. 435; and Re/iertoire, &c., 

 p. 1362. 



X Phil. Trans. 1815, p. 15J. Prop. XXV. 



