Sir David Brewster on certain Affections of the Retina. 28 
have described in the Philosophical Magazine for 1834, have an 
interest of a different kind. When the luminous impressions 
succeed each other at a certain interval, the luminous disc, even 
when small, exhibits a reticulated structure like a mosaic pave- 
ment composed of distinct hexagons delineated in black lines 
(sometimes with a black spot in their centre), indicating that 
those portions of the retina are temporarily insensible to light. 
In order to see this phenomenon distinctly, we must employ 
a large dise uniformly illuminated, such as a glass globe ground 
on both sides, or a plate of ground glass. When the revolving 
dise has a particular velocity, the illumimated surface will be 
seen covered with the hexagonal pattern already mentioned ; and 
as the velocity diminishes, the pattern breaks up, exhibiting por- 
tions of circles and imperfect crosses, the exact forms of which 
it is difficult to describe. In daylight, when the light of the sky 
is used, the colours which accompany these phenomena are 
better seen, and also the variations which they undergo in refer- 
ence to the foramen centrale of the retina, as described in a former 
paper. 
When we maintain the revolving disc at that particular velo- 
eity which produces the hexagonal pattern, so that the retina 
may be greatly excited, a beautiful pattern of a very different 
kind makes its appearance, occasionally mixing itself with the 
first pattern, but most beautifully seen opposite the dark inter- 
vals between the slits of the disc when the velocity has become 
very slow, and the disc is nearly at rest. This pattern, which 
is too evanescent to permit it to be drawn, consists of a series of 
dark quadrangular spaces separated by a triple or multiple line of 
light. It has no relation whatever to the hexagonal pattern, and 
is never seen unless when the eye has been strongly impressed 
by the successive impulses of the luminous disc. 
When we observe these phenomena at different distances from 
the illuminated disc, the hexagons always subtend the same angle, 
which I have found to correspond with a space on the retina 
equal to the 420th part of an mch; and there can be no doubt 
that they are produced by a structure of a hexagonal character. 
In so far as the human retina has yet been studied, no structures 
of a hexagonal character have been discovered. In the choroid 
coat, however, in front of the retina nucleated cells of a slightly 
hexagonal form have been seen in man and in almost all mam- 
malia*; and it is not improbable that the parts of the retina 
immediately behind these hexagons may be so affected by them 
as to produce the hexagonal forms which we have described. 
With regard to the quadrangular pattern, in which the dark 
* Nunneley, ‘On the Organs of Vision,’ p. 171, and plate 2. fig. 7. 
