The document discusses the history of the invention of the achromatic lens, which corrects chromatic aberration by using two lenses made of different glass types with different dispersions. In the early 1700s, British mathematician Chester Hall figured out the formula to correct color this way but did not know if suitable glasses existed. He later discovered by accident that eyeglasses used two different glass types. Hall contracted with two opticians to secretly make prototype lenses to prove his theory, but both subcontracted the work to George Bass, who assembled the lenses and discovered they eliminated chromatic aberration. John Dolland overheard of this and patented the invention, becoming rich.

1. History of the invention of the achromatic lens
David Shafer
David Shafer Optical Design
Fairfield, CT 06824
#203-259-1431
shaferlens@sbcglobal.net

2. To correct chromatic aberration
requires two different glass types,
with different dispersions. In the
early 1700s there were only two
types in wide use – plate glass for
dishes and windows and eye glasses,
and leaded chandelier cut crystal
glass. They have quite different
dispersions but nobody noticed that
or thought it important.
Uncorrected
Corrected
Different
color focii
Same
color focii
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3. Sea captains and pirates were the main users of spyglasses. They had
bad image quality, partly because of uncorrected color due to using a
single glass type. There was a large market potential if someone could
figure out how to fix that image defect. 9

4. Better image quality would allow seeing more detail on far away ships and might
give a military advantage. Chester Hall, a British mathematician in the early 1700s,
figured out the formula for how to correct for color with lenses of different
dispersions, but did not know if there were any such glasses available until he
found out by accident when getting some new eyeglasses made.
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5. Chester Hall wanted to
get an example actually
made to prove his
calculations, using the
two glass types he now
realized had different
dispersions.
But he wanted to keep
this a secret, for a
while. He was afraid if
he gave instructions to
an eye glass maker that
person would might
happen to put the two
finished lenses together
and see the absence of
color, and then steal
the invention.
So Chester Hall contracted with two
different opticians, one for each of the two
lenses, as a precaution to guard his secret.
Edward Scarlett and James Mann
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6. Edward Scarlett and James Mann
George Bass
Both opticians
were very busy so
they subcontracted
the work to the
same man, George
Bass. He made the
lenses, put them
together, and saw
no color. Eureka!
John Dolland
overheard Bass
talking about this.
Dolland right away
filed for a patent
and then got rich.
How is that for
justice?
In 1758
Dolland
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