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» NADIE 
ON ENTOPTIC VISION. 
Parts II. anp III. 
PART II.—THE ENTOPTISCOPE AND ITS APPLICATIONS. 
By W. F. BARRETT, F.RB.S., 
Professor of Experimental Physics, Royal College of Science for Ireland. 
(Puates III., IV.). 
[Read, NovemBer 21, 1905; Received for Publication, NovemBER 24, 1905; 
Published May 2, 1906. | 
Gil, 
In the present paper the instrument which I have devised for 
viewing, delineating, and measuring entoptic objects will be 
described, and some experiments with, and applications of, the 
Entoptiscope will be given, though its practical use must be left 
in the hands of the ophthalmic surgeon. 
Tn this instrument, instead of the troublesome method of double 
vision referred to in Part I., the observer /ooks through the pin-hole 
orifice on to a brightly illuminated ground-glass screen, placed 
upon what corresponds to the stage of a microscope. By this 
means the magnified image of the entoptic object is projected 
upon the ground-glass below, and the shadow can easily be traced 
with a pencil. An important feature of this arrangement is that 
the pencil-point appears to be exactly in the plane of the projected 
shadow, so that no difficulty is experienced in making a tracing. 
The observer, especially if presbyopic, will be astonished to find, 
on looking through the minute orifice, how distinctly he sees the 
sharp point of the pencil, or of any small object in the field of 
view, albeit far within the limit of clear vision of his unaided eye. 
This is due to the homocentric pencil of rays through the pin-hole 
orifice, and has already been explained in Part I., § 5. 
