Barrert—On Entoptic Vision. 59, 
then the lower opening O”, the shadows are seen on the retina 
displaced as shown in the upper and lower projections. The 
entoptic object No. 2 in the plane of the pupil has not changed 
its position, but No. 1 in front of the pupil, and No. 3 behind, 
have both been displaced, but 7 opposite directions. Obviously 
this displacement or parallax is greater as the distance from 
the pupillary plane is greater, and thus the position of an 
obscurity within the eye can be found by the observer with a 
certain degree of accuracy. 
This method, however, is not applicable to moving objects like 
the musce, and in any case requires a considerable amount of skill 
on the part of the observer. Brewster’s method is better, and was 
improved upon by Donders by using two adjacent openings in a 
Bie. 8. 
stenopaic screen: each aperture about 0-1 mm. in diameter and 
about 2 mm. apart. Two overlapping images of the pupil are 
now seen as shown in a 0 (fig. 8). The closer the pin-holes are 
together, the more the circles overlap; the distance apart of the 
centres of the two circles exactly corresponding to that portion of 
the double image which does not overlap: and this distance, when 
the entoptic rays are parallel, is proportional to the distance 
between the pupil and the retina. Ii now an entoptic object, 
such as No. 2 (fig. 8), lie in the plane of the pupil, its doubled 
shadow will be as far apart as the images of the pupil, ze. as the 
centres of the two circles. On the other hand, the doubled 
1 Nederlandsch Lancet, 2nd Series, D. 11,1847. Archiv. f. physiologische Heilhunde, 
viii., 1849. Accommodation and Refraction of the Lye, by F. C. Donders, 1864, p. 203. 
SCIENT. PROC. R.D.S., VOL. XI., NO. VII. K 
