326 Oji the Coloured Houppes or Sectors of Haidlnger. 



As this question is entirely an experimental one, I have sought 

 for an experimentum crucis independent of the preceding consi- 

 derations. The two following experiments appear to me to have 

 this character : — 



1. If in place of transmitting the polarized light through the 

 whole aperture of the pupil, w^e look through a small pin-hole 

 the fiftieth or sixtieth of an inch in diameter, the coloured sec- 

 tors are seen of the same size and form, but only fainter, as with 

 the pupil when its aperture is a maximum. 



2. If we look through the narrowest slit that admits a suffi- 

 cient quantity of light, and give it a motion of rotation in front 

 of the cornea, so that the coloured sectors may be examined 

 when produced by light incident in every azinuith, we shall find 

 that the sectors neither change their position nor their form, 

 and consequently cannot be produced by the refractions which 

 are made by the cornea in every azimuth of polarization. 



If these views be correct, it follows that the structure which 

 produces the coloured sectors must exist at the end of the optical 

 axis of the eye, and in the retina between the vitreous humour 

 and the sclerotic coat. The existence of such a structure can 

 be determined only by anatomists ; and hence the true cause of 

 the coloured sectors cannot be discovered till the structure of the 

 retina is better known. The drawings of this membrane by 

 Brucke and Kollikcr, and of the foramen centraJe by Soemmer- 

 ing, exhibit no combination of polarizing and refracting surfaces 

 capable of explaining the optical phsenomenon ; but in the more 

 recent dissections of J\Ir. Nunneley of Leeds, published a few 

 months ago in his ' Treatise on the Organs of Vision,' there are 

 obvious traces of the required structure. 



I have long ago proved by unquestionable experiments, that 

 there is a part of the retina, corresponding with the sectoi's, and 

 exactly of the same angular magnitude, which becomes sensible 

 to light sooner than the rest of the retina, and shows itself in a 

 dark reddish spot 4i degrees in diameter. This property must 

 be the result of structure; and if we suppose the spot to be 

 covered with a polarizing film, or to have the structure of one, 

 all that we require to explain the phrenomenon of the sectors is 

 a few refractions, accompanied with polarization, like those which 

 take place at the cornea, or with a number of convex or concave 

 surfaces. Now it is a remarkable fact, that in jNIr. Nunneley's 

 drawing of i\M foramen centrale of the retina there is the a})pear- 

 ance of such a structure*; and as this membrane consists of eigJit 

 layers, including structures of different forms, it is probable tliat 

 the refractions which take place at each, small though they must 

 be, may be sufficient to produce the optical phjenonu-non. 

 * Plate 1. fiK. 1!^ 



