Barrett—On Entoptic Vision. 121 
has gone, I have found fair complexions see the yellow spot 
(by means of the oxalate solution) quite as distinctly as persons 
with dark complexions. In Plate VII. the shaded, nearly 
circular patch, marked Y, is the yellow spot as seen by two 
assistants and myself. Mr. Warwick is dark-complexioned, with 
black hair. Mr. Ledwidge is fair, with light hair; and I am, or 
was in youth, more fair than dark. It will be noticed that the 
area of the yellow spot is nearly the same in all, being slightly 
larger in the fairest complexion, Mr. L.; the squares represent 
half millimetres, so that its diameter is about one millim.! The 
horizontal diameter of the macula (as already remarked) is, as a 
rule, slightly greater than the vertical; its boundary is somewhat 
irregular and indistinct, but no attempt was made to reproduce 
this in the drawing. 
In determining the area of the macula, the observer was seated 
about 1 metre from a large sheet of squared paper; anda glass 
trough, with a weak blue-green solution of oxalate of chromium 
and potassium, was placed near to one eye, the other eye being 
closed. The paper was brightly illuminated by the sun, and the 
horizontal and vertical boundary of the dark reddish patch were 
noted by the observer and marked on the paper. These intervals 
were then measured, and also the distance of the paper from the 
eye. Mr. W.has normal sight; Mr. L. slightly myopic; and I am 
presbyopic, but had normal sight until recent years. Taking the 
nodal point as 16 mm. from the retina, except in Mr. L.’s case, 
when 17 mm. was used, as being more nearly correct, the 
horizontal diameter of the macula was found to be nearly a 
millimetre (0‘°9mm.); and the vertical slightly less. The angle 
subtended by the yellow spot is a little over 3°.° 
3. Tue Puncrum Cacum or Buinp Spor. 
It is well known that a lacuna exists in the field of vision 
corresponding to the region where the optic nerve enters the eye ; 
in this region the sensory filaments of the retina are absent. The 
1 Tt may only be a chance coincidence, but the two observers who saw the blue 
sectors in Haidinger’s tufts so very clearly were both persons of very fair complexion 
and light hair. 
* Not ‘‘ 6° to 8°,”’ as some authorities state. 
