Barrett—On Entoptice Vision. 123. 
manner; but there is a considerable difference in the results 
obtained. The angle subtended by the horizontal diameter has 
been found by different observers to vary from 3° 40’ to 9° 40’, 
and the vertical diameter also to vary considerably. The mean 
of numerous measurements made by Mauthner, on different eyes, 
gives 6° as the angle subtended by the horizontal diameter of the 
blind spot ;! this corresponds to a linear diameter of 1:7 mm., 
taking the nodal point as 16°6 mm., or 1-67 if the nodal point 
be taken as 16 mm. This agrees with my own observations 
(see below), and fairly well with the average obtained by other 
observers, and also with the actual measurements made after 
death, or in extracted eyes. 
It occurred to me that it would be desirable to repeat these 
measurements; not only to ascertain the area of the blind spot in 
a few cases, but also and chiefly for the purpose of finding the 
value of the :mportant factor x—the distance of the nodal point 
from the retina. For this purpose the observer was seated at a 
definite distance (about a metre) from a sheet of squared paper 
fixed vertically, and kept one eye (the other being closed) steadily 
fixed on a mark made on the paper. A blackened drawing-pin, 
which was fixed to a wire handle, was then moved outwards 
from the point of fixation until it disappeared from view. 
This point was marked, and the movement was continued until 
it re-appeared, when a second mark was made. ‘The vertical 
diameter was determined in like manner; and the general outline 
of the blind spot was also found. The same three observers as 
before (myself, Mr. Warwick, and Mr. Ledwidge) tried the 
experiment; the result of numerous repetitions closely agreed. 
Using the same letters as above, = tan a; hence, if D be 
k 
taken as 4°2 mm., & can be found by measuring the angle 
between the centres of the blind spot and of the yellow spot, as 
seen projected from the eye. On the other hand, taking x as 16 
in my case and Mr. W.’s, and 17 in the case of Mr. L., the 
distance from the centre of the macula to the centre of the 
_ papilla (the blind spot) came out as 4°2 mm. to 4:25 mm. in all 
three cases. (See B, Plate VII.) 
1 According to Snellen and Landolt, Zraité complete d’Ophthalmologie, vol. i., 
p. 621. 
