perplexities are introduced by the fact that the use of a wide
pencil of light is a desideratum, in order to gain sufficient
illumination when large magnification is to be secured.

In the attempt to overcome those difficulties, the foremost
physical philosophers of the time came to the aid of the best
opticians. Very early in the century, Dr. (afterwards Sir David)
Brewster, the renowned Scotch physicist, suggested that certain
advantages might accrue from the use of such gems as have high
refractive and low dispersive indices, in place of lenses made of
glass. Accordingly lenses were made of diamond, of sapphire, and
so on, and with some measure of success. But 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