Barrett—On Entoptic Vision. 51 
at the magnified image of a scale-division. Obviously, if the 
magnifying power of the microscope is known, the actual size 
of a microscopic object can be thus found. In like manner 
an entoptic object can be sketched, and its magnitude estimated, 
one eye looking through the illuminated pin-hole screen, and 
the other eye viewing the pencil and sheet of paper on which the 
enlarged image is drawn; the distance of the paper from the 
eye being measured, the magnification is the ratio of this distance 
to the distance of the retina from the optical centre of the eye 
(16 mm.). But, owing to the difficulty of getting the plane 
of the image to coincide with the plane of the paper, this 
method is only feasible to a practised observer. A much 
simpler and more accurate method of drawing the entoptic 
image, and one requiring no skill, forms an essential feature of the 
new Entoptiscope to be described in Part II. Moreover, the actual 
magnitude of the entoptic object is also found at once by the 
Entoptiscope without any difficulty. 
§ 5. 
There are two interesting facts in connexion with entoptic 
observation which require a brief explanation. On looking 
through a minute aperture in a card held close to the eye, it 
will be noticed that an object, such as a pencil-point, can be seen 
with perfect distinctness, even when brought within an inch or 
two of the eye, 7.e., far within the limit of clear vision ; and this 
notwithstanding that the observer may be hypermetropic or 
presbyopic. The fact is wel] known, and is sometimes used 
for enabling very small print to be read with ease. The accom- 
panying diagram, fig. 5, will help to explain this anomaly. If a 
small object a, 6, be placed near the eye, the screen with pin-hole 
aperture SS, in the first instance being removed, the pencil 
of rays from the object reaching the eye at m', m’, would 
be refracted and encounter the retina at g, h, and 7, /; the 
prolongation of these rays would unite at ( and a, when 
a clear inverted image of the object would be formed and 
perceived, if the retina were at that distance. As it is, how- 
ever, a blurred and wholly indistinct image is formed on the 
