Barrett—On Entoptic Vision. 119 
the major axis being horizontal, and in like manner the fascicula 
or tufts appear somewhat longer when seen in a horizontal than 
in a vertical plane. 
Knowing the distance of the nodal point from the retina,’ the 
retinal area covered by the tufts can be found by measuring the 
size of the projected image and its distance from the eye. This 
does not appear to have been hitherto measured, nor can I find 
any similar subjective measurements of the yellow spot. I have 
therefore obtained several determinations of both the macula and 
the fascicula from different observers. ‘The general result gives 
the vertical diameter of the yellow spot, and also the vertical axis 
of Haidinger’s tufts on the retina as about 0°8 mm.; the horizontal 
diameter in both cases is a little greater, going up about one mm. 
Post-mortem measurements of the yellow spot are found to vary 
according to the subject. Kolliker found the vertical diameter 0°8 
mm., and the horizontal considerably greater; but E. H. Weber 
made the horizontal diameter 0°8 mm.; so that our subjective 
measurements may be taken as agreeing with the objective 
measurements made after death, or on extirpated eyes. The area 
of the fovea is, however, much less than that of the yellow spot; 
its greatest length does not exceed 0°2 mm. Accordingly, the 
view taken by Helmholtz and Brewster, and hitherto generally 
accepted, that Haidinger’s tufts are restricted to the fovea centralis, 
is incorrect : their extent is four times greater, and corresponds, as 
I have said, to the limit of the yellow spot. 
Now, throughout the yellow spot the rods of the bacillary 
layer of the retina are absent; the cones only are present; and it 
would seem as if some connexion existed between this special 
structure and the perception of the tufts—that is to say, the detec- 
tion of polarized light and cf the plane of polarization by the 
unaided retina. 
2. Tur Macuta Lutes or YELLOW Spot. 
It is a singular fact that the region of most distinct vision on 
the retina should be coloured yellow. One would have thought 
that this would seriously interfere with colour-perception, especially 
of blue and green; but we are not conscious of this under ordinary 
1 See Part I., Proc. Roy. Dublin Soc., vol. xi., p. 53. 
