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Yes I have first hand experience of this phenomenon. When I bought my first
sextant, a Plath Professional it arrived from Germany and I began the
arduous process of trying to teach myself how to use the instrument. I had
massive problems as every time I pushed the sunshades down and looked at the
sun the shades did not cover the total area of the telescope and I would get
a blast of pure Australian sunshine in the right eye. This was particularly
the case when I was scanning back and forth trying to pick up the sun at
noon. I tried this several times getting very sore eyes as a result,
particularly the right eye.
Finally I sat down with the instruction manual and discovered that the
sextant had been delivered with the shades transposed ie the horizon shades
had been fitted on the index mount and the index shades on the horizon
mount. On a Plath Professional this is quite an error as the plastic shade
housings look the same but are in fact of different sizes. The Plath went
back in the box and made the long trip back to the German factory.
I was very concerned and consulted one of Sydney's best eye doctors - a
specialist Ophthalmologist. He subjected me to a battery of tests
particularly my right eye, looking for marks of scarring on the retina.
Result - totally clean. He said that the most common cause of retinal
scarring is with drug addicts who lie prone and gaze at the sun for long
periods. He said it takes some time to burn the retina and that a normal
person would find it hard to stare at the sun long enough to cause damage,
as it becomes very painful. A comatose drug addict can do it because they
are partly stupefied.  Instinctively, humans have evolved to turn away from
the sun after only a very brief exposure (in his opinion). The blink
response is part of this defence mechanism apparently as is the tendency in
some people (me included) to sneeze when your face is turned into strong
sunlight.
The ophthalmologist did not believe that the flashes I was getting around
the edge of the shades would be enough to cause damage even though it made
my eyes sore and left, temporarily, a green halo in the periphery of vision
of my right eye. Also I was not using sunglasses at that stage which I do
now, but he held to his opinion that the brief flashes in the telescope,
although they should have been avoided, had not caused harm.
When I asked him about the blindness experienced by old sailors and many
inland Australian explorers and surveyors he said it could have come from
deficiencies in the composition of the shades but more likely from a whole
host of other factors such as poor diet, vitamin deficiencies or ophthalmic
disease. The Australian disease Sandy Blight or Ophthalmia (a form of
trachoma) which sent a great many of our early explorers blind was an
infectious disease spread by flies and exacerbated by poor diet, glare
irritating the eye and poor optic hygiene resulting from extended periods
without water.. It is now known that it was not caused by the use of
sextants, which was one 19th century theory. Yes men such as Stuart and
Sturt suffered from the disease and they used the instrument constantly.
However a lot of their men who did not know one end of a sextant from the
other also went blind from the disease.
Blindness from Sandy Blight is still a chronic problem among Australian
aborigines today.
Hope this helps.
Kieran Kelly
Sydney
Australia

   

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