Barrett—On Entoptic Vision. 57 
and of any obscurity within the eyeball does not follow the usual 
law of projected images stated above. Singularly enough, this 
important fact seems to have escaped observation hitherto, and I 
must reserve for a subsequent paper the series of measurements I 
have made that demonstrate the law in this case.1 This law the 
Entoptiscope not only determines with precision, but also enables 
one accurately to investigate other obscure phenomena in thie 
psycho-physiology of vision, such as ocular parallax, &c. 
It will be obvious from an inspection of fig. 5 (p. 52) that when 
entoptic objects are seen through a stenopaic screen, if the lines of 
visible direction crossed at 4, the nodal point of the eye, then :— 
(i) the rays would again cross at the pin-hole aperture, and the 
image would be re-inverted (that is, it would be seen upright); but 
this is not the case; and (ii) the enlarged image would be projected 
on to the surface of the stenopaic screen and its apparent magnitude 
Fie. 6. 
would follow the usual law, but neither of these is the case. 
In fact, the path of the projected shadow corresponds to the 
path of the incident-rays crossing at the pin-hole aperture only. 
This is shown at O in fig. 6. Hence the magnification is, as 
might be inferred a@ priori, in the direct ratio of the distance d of 
the pin-hole from the pupil to its distance D from the illuminated 
screen Son which the shadow is projected. ‘The pupil p forms the 
base ab of the cone of rays on one side, and the projected image 
of the pupil 4 B forms the base of the cone of rays on the other 
side. When O is at the anterior focus of the eye, the retinal 
1 Even Helmholtz and Donders, and more recent authorities on physiological optics 
such as Tscherning (Physiologic Optics, Eng. Trans., p. 304), appear to have overlooked 
the obvious considerations stated in the next paragraph, and accordingly have given 
an incorrect formula for calculating the size of entoptic objects. 
