DEPARTMENT OF GENERAL PHYSICS. 507 



lliey origiuuto Laa long been the subject of controversy, and will be discussed 

 presently. 



The longer axis of the yellow tufts, or hour-glas8, coincides with the plane of 

 polarisation of the incident light, and therefore rotates with the rotation of the 

 Nicol. If the Nicol prism be kept unmoved before the eye the appearance 

 vanishes in a few seconds ; when the Nicol is suddenly turned to a difterent plane 

 the tufts reappear, fading again in a few seconds if the Nicol be unmoved. This 

 fugitive appearance is characteristic of several other entoptic phenomena, and 

 explains why so many observers have failed to see Ilaidinger's tufts. Sir D. 

 Brewster states that he tried in vain for over twelve months before he saw them ; 

 von ITelmholtz remarks that in spite of the greatest efforts it was twelve vears 

 before he perceived them. There is not, however, the least difficulty in seeing 

 them if the following details are attended to. Choose a bright sky or a sheet of 

 paper brightly illuminated with white light ; hold the Nicol before one eye. and 

 close the other; turn the Nicol backwards and forwards every two or three seconds, 

 and the tufts will be seen projected on the illuminated surface at the extremity of 

 the optic axis. If the observer still fails to see them, hold a piece of cobalt-blue 

 glass in front of the Nicol, and a dark-reddish hour-glass-shaped patch will be 

 distinctly seen. The cobalt blue transmits the extreme red rays. By keeping the 

 Nicol in slow rotation the tufts can be continuously seen. 



When the tufts are seen projected on a white surface, if the plane of polarisa- 

 tion be horizontal and the Nicol suddenly removed from the eye, the tufts will 

 still be seen faintly, but in a vertical plane. This is no doubt due to the partial 

 polarisation of the light reflected from the white surface, for the effect is more 

 marked if a polished black surface be employed. The light of the sky is polarised 

 90° from the sun ; hence those familiar with the appearance of the tufts can see 

 them, without the aid of a Nicol, by looking at a bright sky in a position at 

 right angles to the sun, or at any non-metallic polished surface. The unaided 

 eye can thus detect not only polarised light, but also determine its plane of 

 polarisation. 



At the British Association meeting in 1850 Sir G. Stokes showed that the 

 tufts could not be seen in light of lower refrangibility than the green. If a 

 spectrum be projected on a white surface and the colours successively looked at 

 from the red to the violet through a Nicol's prism, the tufts w-ill first appear in 

 the green, about the F line, and remain visible, if the Nicol be kept moving, imtil 

 the limit of the visible spectrum is reached. 



The origin of Ilaidinger's tufts has been a subject of considerable discussion. 

 There must obviously be some medium in the eye which is able to act as an analyser 

 to polarised light. Silbermann in 1846 showed that both the cornea and the 

 crystalline lens were bi-refracting, and he concluded that the lens was the source 

 of the tufts. But Zokalski, an eminent Paris oculist, subsequently found that 

 four aphakic patients (those whose lens had been removed) were able to see Haid- 

 inger's tufts. Jamin, in 1848, urged the corneal origin of the tufts, and succeeded 

 in fairly well reproducing their shape by making an artificial cornea of a pile of 

 watch-glasses pressed together, and examining the system by polarised light. 

 But Brewster, in a little known but valuable paper in the ' Comptes Rendus ' for 

 1859, conclusively proved that the cornea could not be the seat of the appearance. 

 For there was no alteration iu the size or shape of the tufts when an opaque screen 

 with a very minute circular aperture was held before the eye, nor when a narrow 

 slit was used and slowly rotated before the eye. The tufts were seen more faintlv, 

 but that was all. 



Brewster maintained that the origin of the tufts must be sought for in the 

 region of the eye at the extremity of the optic axis, and concluded that their cause 

 was due to some bi-refracting property possessed by the fovea centralis of the 

 retina. This was also the view held later on by Helmholtz, who pointed out 

 that many organic fibres and membranes are bi-refracting, and act in general like 

 uniaxial crystals. The production of the tufts could therefore be explained by 

 assuming that the fibrous layer of the macula (the yellow spot) was bi-refracting, 

 absorbing the extraordinary ray more strongly than the ordinary. The peculiar 



