Barretr—On Entoptie Vision. 77 
(a). Look through one eye-piece; bring a pencil-point as at 
6, fig. 8, to the very edge of the projected pupillary disc, looking 
directly at the pencil. Without moving the pencil or the head, 
turn the eye to the opposite edge a of the pupillary disc; the 
pupil will appear to have suddenly expanded to 8’, as shown by the 
dotted lines; and the pencil will, therefore, be now seen well within 
the disc; but when again the eye turns to 3, the disc resumes its 
first position on that side, and the side a correspondingly expands. 
(b). Now shift the pencil outside the disc, between 0 and 0’, as 
shown by the dot, so that it cannot be seen if directly searched 
for ; when, however, the eye is turned away from it to a, instantly 
it reappears ; this occurs equally well at either side of the dise, with 
either eye. ‘This paradoxical effect of seeing an object when you look 
away from it, and not seeing it when you look directly towards it, is 
not due to the image falling on the blind spot, for it occurs, as I 
have said, at either side of the disc, or at any portion of the retina 
oblique to the axis of vision.’ 
By making pencil-marks at the limiting points at which the 
pencil is seen by direct and oblique vision, the magnitude and 
published by Sir Dayid Brewster in the Trans. Roy. Soc. Edinb. for 1848, vol. xv., 
p- 349, contains the first reference I can find to ocular parallax, which Brewster 
appears to have observed in cases of oblique incidence in ordinary vision. Subsequently 
Listing investigated the matter, Beitrag zur Physiologischen Optik, 1845, pp. 14 et 
seq., and gave the probable explanation (see next footnote). 
1 The cause of this ocular parallax I will discuss in a subsequent paper. Brewster 
assigns it to the fact that the eye is not a homogeneous refracting medium; Listing 
to the fact that the centre of rotation of the eyeball does not correspond to the optical 
centre or nodal point of the eye. Helmholtz (Optigque Physiologique, p. 748) agrees 
with and develops Listing’s view, but is unaware of Brewster’s earlier discovery and 
explanation. None of those, nor any later authorities, however, appear to have noticed 
the remarkable effect of this parallax in entoptic observation with a stenopic screen. 
SCIENT. PROC. R.D.S., VOL, XI., NO. IX. M 
