Barrett—On Entoptic Vision. 117 
The Entoptiscope, described in my former paper,! affords a 
ready and conclusive means of determining whether the tufts are 
due to any of the refracting media in the eye. Using a double 
pin-hole aperture in the diaphragm, each pin-hole being 0°'4 mm. 
in diameter, and the two separated by a space of 1°5 mm., the 
tufts can be seen when a good light is used and a Nicol’s prism 
placed before the diaphragm. Ifthe tufts are solely caused by any 
structure in front of the pupil, a double image of the tufts will be 
seen more widely separated than the centres of the duplicated 
pupillary disc; if due to the crystalline lens or vitreous humour, 
a double image of the tufts will be seen /ess widely separated than 
the pupillary discs. Experiment, however, shows that only a single 
image of the tufts is perceived ; their origin is, therefore, in the 
retina itself. This was the view held by Brewster, who maintains 
that the origin of the tufts must be sought for in a region at the 
extremity of the optic axis, and suggests that their cause may be 
due to some peculiar bi-refracting property possessed by the 
JSoramen centrale (the fovea centralis) of the retina.’ Haidinger, in a 
letter to Sir G. Stokes, first published in Stokes’s collected papers,’ 
also takes this view, and (apparently unaware of Brewster’s paper) 
shows that Jamin’s theory and experiments do not meet the facts. 
Helmholtz! strongly supports the retinal origin of the tufts, 
though he makes no reference to Brewster’s paper. Helmholtz 
points out that many organic fibres and membranes are bi-refracting, 
and in general act like uniaxial crystals, the axis being parallel 
to the direction of the fibres, or at right angles to the surface of 
the membrane. The production of the tufts could thus be explained 
by assuming that the fibrous layer of the macula lutea (the yellow 
spot) is bi-refracting, absorbing the extraordinary ray more strongly 
than the ordinary. The peculiar shape of the tufts, Helmholtz 
suggests, may be due to the direction of the fibres in the fovea 
centralis, which constitutes the small retinal depression in the 
centre of the yellow spot. He goes on to remark (quoting from 
the French edition) :— 
‘* Ainsi, d’aprés ’hypothése que nous avons faite, un bord de la 
fovea, ou les fibres ont en général une direction oblique dirigée vers 
1 See Proc. Roy. Dublin Soc., vol. xi., pp. 60 and 78. 
* Brewster, loc. cit., also ‘‘ Treatise on Optics,” p. 249. 
3 «* Mathematical and Physical Papers,’’ by Sir G. Stokes, vol. iv., p. 60. 
4 Optique Physiologique, p. 553. 
