Newton and the Sun

The name of Isaac Newton is commonly associated with the Moon, 
because of the role it apparently played in his discovery of universal
gravitation, and how much of the Principia was devoted to determining 
the characteristics of the Moon's orbit, and so on.  Newton is supposed
to have said that his head never ached except for his studies of the
Moon.  That may be true, but it seems that he had grappled with at
least one other astronomical body early in his career, when he was
studying optics.

One of the things that first interested Newton in optics was the 
phenomenon of "fantasy", which is the tendancy to continue "seeing" 
a bright light for awhile after the light goes away, even if you 
close your eyes.  To test this effect, he looked with one eye directly
at the Sun for an extended period of time (!) until (as he reports 
in his notes) everything appeared either pale red or blue.  Then he
went indoors, and after "ye motion of ye spirits in my eye were 
almost decayed" (i.e., normal vision was returning to the eye that
had been exposed to the Sun), he closed that eye and, by imagining
the Sun, found that the bright spot seemed to reappear to his vision.
Then when he opened that eye he found that everything looked either 
red or blue again, i.e., the "spirits" of his eye had been stirred 
up again.  Incredibly, he seems to have undertaken this experiment
without considering the possibility of permanently damaging his 
eyesight.  Only when the spots and optical "fantasies" persisted did
he become worried, and he shut himself up in total darkness for several
days before they finally subsided and his normal vision returned.  It
isn't clear if he told anyone what he had done, or if anyone noticed
him sitting in his darkened rooms.

In his subsequent optical experiments Newton was more cautious about
looking at direct sunlight, but he was still remarkably reckless in
is treatment of his own eyes.  For example, about a year later he
wedged a blade "betwixt my eye and ye bone as near to ye back side 
of my eye as I could" to see the effects of deforming the retina.
He even included a scary drawing of this experiment along with 
the written description in his notes.  These activities, combined 
with all the poisonous chemicals (like Mercury) that he inhaled 
during his chemical experiments, make it surprising that he lived
in such good health to such an old age.  It's reported that when
he died at the age of 84 he still had all his teeth, which would
be unusual even today.