Barrerr—On Entoptic Vision. 131 
Like other faint Entoptic phenomena, they disappear after gazing 
at them for a little while, but can be revived at intervals as often 
as one pleases, a very brief rest being sufficient to renew the 
sensitiveness of the retina. We have attempted to reproduce 
what is seen in fig. 1, Plate VIII., though the specks are in many 
cases mere points, and far more striking and beautiful than can 
be represented in black and white.! 
In order to estimate their size, a sheet of millimetre ruled 
paper was fixed vertically in the sunlight at a given distance from 
the eye; and numerous observations independently made by 
myself, Mr. Ledwidge, and other assistants. Taking the nodal 
point as 16 mm. from the retina, the retinal image of the moving 
corpuscles was found to be from ‘009 to ‘014 according to 
different observers. Assuming the corpuscles to be very close to 
the sensitive layer of the retina, this would give their approximate 
diameter, the mean result being about ‘611 mm., or say the 3,5,th 
of an inch. 
A better way of estimating their size I found to be as 
follows :—Pinholes a millimetre in diameter were made in a small 
strip of thin metal, which was then fixed to a window-pane facing 
a bright sun-lit sky. Looking at the pane of glass through two 
thicknesses of cobalt-blue glass, the moving corpuscles were 
seen, and the observer moved to and fro until the diameter of 
the bright specks appeared to be the same as that of the 
bright apertures in the little opaque screen. This occurred when 
the eye was from 120 to 160 cm. from the window-pane: the 
retinal image is therefore +> to +1, of a millimetre in diameter, 
and the corpuscles may be taken as about this size, though 
their diameter varies, and the result can only be regarded as 
approximate. 
That the phenomenon is associated with the circulation in the 
retinal vessels appears unquestionable ; but that the appearance is 
due to rolls of red blood-corpuscles seems very unlikely. The 
enormous number of these corpuscles in the blood is one reason 
1 The drawing gives a better idea of the drift of the corpuscles if it is looked at 
after turning the page through a right angle. 
* Their size may also be estimated by comparing them with the thickness of any 
definite musca seen projected on the same plane : it will be observed that their diameter 
is far smaller than the width of any of the musce volitantes. 
