60 Scientific Proceedings, Royal Dublin Society. 
shadow, if the object le in front of the pupil, as No. 1, will be 
further apart than the centres of the two circles, and if between the 
pupil and the retina, as No. 3, the shadows will be closer together. 
It is now only necessary to find the breadth of the part of the 
circles which do not overlap in order to find the relative position 
of an obscurity in the eyeball. For the distance apart S of the 
duplicate shadow is in the same ratio to the distance R of the 
entoptic object from the retina, as the distance apart C of the 
centres of the two circles is to the distance P of the pupillary 
plane from the retina; or 
Si, S 
R Pp hence R= GP. 
Now the distance P, of the pupil from the retina in a normal 
eye, may be taken as 19 mm., in a myopic eye about 21 mm., 
and in a hypermetropic eye about 17mm. Hence the exact 
position of the entoptic object in the eyeball can be located. 
Fie. 9. 
This is more clearly seen in fig. 9, where A and B are the over- 
lapping images of the pupil; and the obscurities, in the order of 
their distance from the cornea, are diagrammatically represented 
by a triangle, a square, and a circle. As both figs. 8 and 9 
are accurately drawn to scale, by using a pair of compasses, 
the reader can verify the law above stated. In connexion with 
the Entoptiscope to be described in Part II., I will show how this ~ 
