24 Sir David Brewster on certain Affections of the Retina. 
quadrangles are separated by several parallel bright lines, it is 
obviously produced by some structure in the retina itself; and 
it is possible that it may arise from some regular arrangement 
of the rods in the columnar or bacillar layer which has not yet 
been detected by the microscope. 
The hexagonal pattern 1s very distinctly seen in the flame of 
a coal or a wood fire, at that particular part of it where the jets 
of ignited gas succeed each other at the proper interval. 
After making these experiments for some days in succession, 
I have found that the hexagonal pattern, and sometimes even the 
uadrangular one, is seen when the eye is accidentally directed 
to faintly illuminated surfaces. This fact seems to show that 
the pattern is rendered visible merely by the excitement of the 
retina with the action of hght, and not by its successive impulses, 
the black lines of the hexagons and the dark spaces of the triangles 
requiring a longer time to exiibit the action of the faint light 
than the other parts of the retina—a property which I have found 
at the foramen centrale, and at various points of the retina at or 
near the ora serrata, its anterior margin. 
As an inducement to optical observers to investigate any of the 
abnormal phenomena of vision which may come under their 
notice, I give the followimg lst of properties or structures in the 
retina which have been discovered, or are manifested, by optical 
observations. 
1. The polarizing structure which produces Haidinger’s brushes. 
2. The insensibility of the retina at the entrance of the optic 
nerve. 
3. The exhibition of the foramen centrale by its inferior sensi- 
bility to feeble light. 
4, The different sensibility to light possessed by different 
parts of the retina. 
5. The mability of the retina beyond the foramen to maintain 
a sustained vision of objects. 
6. The increased luminosity of objects seen indirectly, or by 
the parts of the retina beyond the foramen. 
7. The hexagonal and quadrangular structure described in 
the preceding pages. 
To these we may add the existence, in the vitreous humour, 
of the remains of vessels no longer required for its support, and 
the existence of cells in the same humour, as proved by the phe- 
nomena of Muscze volitantes, formerly described in this Ma- 
gazine. 
Allerly by Melrose, 
November 30, 1860. 
