Barrett—On Entoptic Vision. 127 
The amount of displacement may be used, and has in fact been 
used, to estimate the distance between the retinal vessels and that 
part of the retina where visual impressions originate. This will 
become clear from the accompanying diagram, fig. 1, which 
shows how Purkinje’s are formed when the illumination is derived 
from a bright spot of light thrown on the sclerotic. If a be the 
illuminated point falling on the sclerotic (it should be shown 
much nearer the cornea), the shadow of a retinal vessel is thrown 
directly on the retina at the point a, and thence projected, through 
the nodal point /, to some external position a, where an inverted 
image is seen. If the point of light be moved to 4, the projected 
image will move to B, or in the same direction as the light. 
‘Bo 
a ps 
HrGerl. 
If now the distance of the screen on which the image is pro- 
jected from the eye be known, and the displacement of the 
image A B be measured, the retinal displacement a (3 can be 
found as the distance of the nodal point & from the retina is 
known. Ifa particular retinal vessel be seen displaced, the rays 
must intersect it as shown in the diagram; now the distance of 
the vessel from the sclerotic where a and } impinge can be found, 
if the position where the rays a and } impinge be known, and 
accordingly the distance of the retinal vessel from the deeper part 
of the retina whence the visual impression originates can be 
calculated. ‘‘ When this is done, it is found that this distance 
corresponds fairly well to the distance of the retinal vessels from 
SCIENT. PROC. R.D.S., VOL. XI., NO. XII. R 
