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in 1812 a much more important innovation was introduced by Dr. William
Hyde Wollaston, one of the greatest and most versatile, and, since
the death of Cavendish, by far the most eccentric of English natural
philosophers. This was the suggestion to use two plano-convex
lenses, placed at a prescribed distance apart, in lieu of the single
double-convex lens generally used. This combination largely overcame
the spherical aberration, and it gained immediate fame as the "Wollaston
doublet."

To obviate loss of light in such a doublet from increase of reflecting
surfaces, Dr. Brewster suggested filling the interspace between the two
lenses with a cement having the same index of refraction as the lenses
themselves--an improvement of manifest advantage. An improvement yet
more important was made by Dr. Wollaston himself in the introduction of
the diaphragm to limit the field of vision between the lenses, instead
of in front of the anterior lens. A pair of lenses thus equipped Dr.
Wollaston called the periscopic microscope. Dr. Brewster suggested that
in such a lens the same object might be attained with greater ease by
grinding an equatorial groove about a thick or globular lens and filling
the groove with an opaque cement. This arrangement found much favor,
and came subsequently to be known as a Coddington lens, though Mr.
Coddington laid no claim to being its inventor.

Sir John Herschel, another of the very great physicists of the time,
also gave attention to the problem of improving the microscope, and in
1821 he introduced what was called an aplanatic combination of lenses,
in which, as the name implies, the spherical aberration was largely
done away with. It was thought that the use of this Herschel aplanatic
combination as an eyepiece, combined with the Wollaston doublet for the
objective, came as near perfection as the compound microscope was likely
soon to come. But in reality the instrument thus constructed, though
doubtless superior to any predecessor, was so defective that for
practical purposes the simple microscope, such as the doublet or the
Coddington, was preferable to the more complicated one.

Many opticians, indeed, quite despaired of ever being able to make a
satisfactory refracting compound microscope, and some of them had taken
up anew Sir Isaac Newton's suggestion in reference to a reflecting
microscope. In particular, Professor Giovanni Battista Amici, a very
famous mathematician and practical opti