48 Scientific Proceedings, Royal Dublin Society. 
will now be clearly depicted on the focussing screen, but will 
disappear as soon as the image of the pin-hole is once more focussed 
on the screen. All this is well known and readily explicable, 
but the exactly similar results that occur in our own eye are not so 
well known. When the illuminated pin-hole is placed at such a 
distance from the eye as to be within the power of accommodation of 
the eye, usually a distance of about 9 inches or a foot, a sharp 
image of the pin-hole is focussed on the retina, and no shadow of 
any small object in or near the crystalline lens can be perceived. 
When, however, the pin-hole is brought close to the eye, accommo- 
dation fails to focus its image on the retina, and the shadow of any 
object in the path of the rays will be clearly seen, and the more 
sharply the smaller the pin-hole; whilst the circle of light seen on 
the retina is now not the magnified image of the pin-hole, but the 
shadow of the iris diaphragm of the eye, as has been already 
stated." 
To those who, like myself, suffer from presbyopia, and have 
lost the power of accommodation for near objects, entoptic observa- 
tion can be carried on with a large pin-hole, say, 1 mm. in diameter, 
or any bright spot of light placed at a considerable distance from 
the naked eye; inmy case up to 20 inches. ‘Though the entoptic 
shadows are now smaller, they are darker and less detailed, and 
hence their outline can be more easily drawn under such circum- 
stances. ‘Ilo exclude extraneous light, a sliding brass tube, 
blackened inside, can be made to screw over the pin-hole aperture, so 
that the observer can adjust the length of this tube, through which 
he views the aperture. 
1 Entoptic observation was evidently not known in Shakespeare’s day. I am 
indebted to my friend Canon Elliott for drawing my attention to the following apt 
quotation : — 
Cassius. ‘‘'Tell me, good Brutus, can you see your face? 
Brutus. No, Cassius ; for the eye sees not itself, 
But by reflection by [from] some other things. 
Cassius. ’Tis just ; 
And it is very much lamented, Brutus, 
That you have no such mirrors as will turn 
Your hidden worthiness into your eye, 
That you might see your shadow.” 
Julius Cesar, Act i., sc. 2. 
