Barretr—On Entoptic Vision. 49 
Of course if the presbyopic observer puts on his spectacles, the 
distant pin-hole is now focussed on the retina, and entoptic observa- 
tion ceases until the aperture is brought so close to the eye that 
even with spectacles its image cannot be formed on the retina. 
Ceteris paribus the same thing holds true with myopic vision ; 
hence spectacles of any kind are of no use in entoptic observation, 
and may entirely destroy the effect sought for. 
g 4. 
As the anterior principal focus of the eye may be taken as 
14mm. distant from the cornea, when the pin-hole is placed at 
this distance, the divergent cone of rays entering the eye will 
be rendered parallel, and hence the shadow of any object in or 
between the crystalline lens and the retina will be practically of 
the same size as the object itself. Obviously, when the illumi- 
nated pin-hole is placed nearer to, or further from, the eye than 
the anterior focus, the shadows will be respectively larger or 
smaller than the object.' 
ne, Ye 
This is shown in the accompanying diagrams, where fig. 2 
represents the luminous point a (or stenopaic screen s, s), 
placed at the anterior focus, the enlarged entoptic object 0 
throwing the shadow (3; the rays being parallel within the 
eyeball, the area of the shadow is the same as that of the 
object. In fig. 3 the luminous point a is placed nearer to, 
and in fig. 4 further from, the eye than the anterior focus /; 
in the former the rays will diverge as if proceeding from the virtual 
1 The effect of accommodation, it istrue, renders the position of the principal focus 
of the eye slightly variable ; and, in the case of looking at a pin-hole near the eye, the 
principal focus would be somewhat less than that given as accommodation comes 
into play. The pin-hole, however, should not be looked at but looked through, and 
the eye focussed on the distant surface on which the projected image is depicted. 
