A Cooke triplet comprises a negative flint glass element in the centre with a crown glass element on each side.
It was at that time a major advancement in lens design. The triplet design was made obsolete by later designs on high end cameras, but remained widely used up to this day on cheap cameras.
Despite the fact that the Cooke design was patented in 1893 it seems that the use of achromatic triplet designs in astronomy appeared as early as 1765. The 1911 encyclopedia Britanica wrote:
See also Chromatic aberration
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