From DOUGLAS DICKIE
I am sorry to deprive G. S. Holister of his place in the history of visual
science (Letters, 29 May) but the effect which he describes is a well-known
one.
As an optician, I am frequently asked by patients, particularly children,
about the tree-like pattern which they observe while I am using my
ophthalmoscope. It was indeed his retinal blood vessels (or rather their
shadows) which Holister was observing, but the effect derives from the
remarkable structure of the human eye.
The blood vessels of the retina actually lie in front of the photoreceptors
(an arrangement which surely confirms a Darwinian rather than divine origin
of the eye). What is remarkable then is not that the retinal vessels are
visible under certain lighting conditions, but that we are all not
continuously aware of their presence.
It is the ability of the eye-brain system to filter out such fixed images
that prevents our visual world being confused by superfluous information.
When light is shone obliquely into the eye, different photoreceptors from
those normally used are involved and the retinal vessel ‘tree’ becomes
visible, particularly in the dim lighting of an optician’s consulting room.
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Douglas Dickie
Aberdeen
