Barrert—On Entoptic Vision. 61 
measurement can be made in actual practice; and will also de- 
scribe another and very simple method I have recently devised, 
which enables an unskilled observer to make an approximate 
estimate of the position of the obscurities. 
§ 10. 
By making a large artificial eye, and using a brilliant point of 
light, it is possible to illustrate in a lecture-room the whole of the 
principles of entoptic observation and measurement. This was 
shown at the conclusion of the paper, a small arrow painted on a 
strip of thin glass or mica being used to represent the obscurity 
within the eye and placed in different positions in the imaginary 
eyeball; the shadow of this object being thrown on a ground-glass 
screen to represent the retina. The source of light was an electric 
lamp the rays from which passed through an orifice (about a 
mm. in diameter) in an opaque screen placed before the artificial 
eye. When the ground-glass screen was moved out of focus, the 
sharp shadow of the arrow was seen wherever it was placed in the 
eyeball. The effect of varying the size of the orifice in the opaque 
screen upon the visibility of the shadow of a small object, held at 
different parts of the eyeball, can thus be strikingly demonstrated 
to an audience. 
With two closely adjacent pin-holes two partly overlapping 
circles of light were thrown on the screen; the distance apart of 
the double shadow of the object thus caused was seen to be exactly 
the same as the distance apart of the centres of the two circles, 
when the object was in the plane of the pupil; but this distance 
diminished in proportion as the object approached the retina. In 
fact, the exact distance of the object from the artificial retina could 
thus be readily determined in a lecture experiment. 
SCIENT. PROC. R.D.S. VOL. XI., NO. VII. L 
