Barretrt—On Entoptic Vision. 47 
drawings in rough outline exactly corresponded with what I 
had previously drawn in much more minute detail from entoptic 
observation, only the figures I had drawn were taken from 
inverted images of the opacity in each lens. These drawings, 
and the reason for this inversion, will be given in Part IT.’ 
These results led me to make a series of entoptic observations 
at stated intervals, and ultimately to the construction of a simple 
instrument for the more convenient drawing and measurement of 
what isobserved. Thisinstrument I propose to callan Entoptiscope, 
and it will be described in Part II., as some improvements I have 
recently made are now in process of construction. I hope that 
this instrument may prove a convenient and useful addition to 
the means already employed for the diagnosis and pathological 
study of the eye. 
It may be asked how entoptic observation can reveal obscurities 
in the crystalline lens, when, as is well known, if an ordinary lens 
be partly covered or cut away, or a small opaque object be 
interposed near it, or a fly walk over the lens, no shadow of the 
object nor any injury to the image is seen at the focus of the 
lens. But both theory and experiment show that this is not the 
case when the image is out of focus; a blurred shadow of any 
opaque object in or near the lens is now seen, and this shadow 
becomes distinct and sharp, because free from penumbra, if the 
lens be illuminated by a light from a point, such as a pin-hole 
aperture. A simple experiment with a photographic camera will 
illustrate this. Under ordinary circumstances the diaphragm in 
front of the lens is not seen on the focussing screen ; but throw the 
screen out of focus and allow a beam of sunlight passing through a 
small aperture in a piece of card to illuminate the lens, and the image 
now seen on the focussing screen will be that of the diaphragm and 
will alter in size withit. The shadow of a fly walking on the lens 
1 IT wish to acknowledge my indebtedness to Dr. Charles Fitzgerald for first 
drawing my attention to the chapter on entoptic observation in Donders’ treatise 
on the eye; and also for the yery kind interest which both he and Dr. Benson have 
taken in this investigation. 
