54 Scientific Proceedings, Royal Dublin Society. 
they do not sensibly differ from a single point. This is situated 
on the optic axis close to the posterior (the inner) face of the 
crystalline lens, about 7 mm. from the surface of the cornea, 
and 16 mm. from the retina.' At this point, the object O and its 
retinal image 7 subtend the same angle, and their relative magni- 
tudes are in the direct ratio of their relative distances D and d 
from this point, or, 
Hence, as 7 = = O, the retinal image formed of a metre-stick held 
1-6 metre from the nodal point (7.e., 1593 mm. from the cornea) 
will be exactly 1 em. long. 
What determines the lines of visible direction in the case of 
entoptic phenomena opens up some interesting questions in relation 
to vision, which I propose to consider in a subsequent note. 
Sie 
Assuming that the perception of entoptic objects near the retina, 
when no stenopaic screen is used, follows the same law of projection 
as that of external objects—and experiments indicate that this is 
the case—an obscurity 1 mm. in diameter near the retina will 
be perceived as an inverted image 1 cm. in diameter at a distance 
of 16 cm. from the nodal point, or 15°3 cm. from the cornea. 
Hence, the linear magnitude x, of an obscurity within the eye, 
is to the linear magnitude of the projected image S, as the distances 
d and D of the nodal point from the retina and from the projected 
image respectively, or 
no 5 8. 
It is, therefore, quite easy for anyone to determine for himself, 
without using instrumental appliances, the magnitude of any 
small obscurities within the eye. For example, the musce voli- 
tantes, which float near the retina, throw shadows that are 
1 The position of the nodal point is deduced from the curvatures and indices of the 
refractive media of the eye. Dunders and Helmholtz placed the nodal point 15 mm. 
from the retina of a normal eye; but recent determinations of the refractive index of 
the lens, &c., give a value more nearly like that obtained by Dr. Young a century ago. 
