Sentence:  The <WSD>doublet</WSD> generally used is that invented by Dr. Wollaston, and consists of two plano-convex lenses placed with their convex sides towards the eye ... 
Meaning of the word marked with <WSD> tags:  An arrangement of two lenses for a microscope, designed to correct spherical aberration and chromatic dispersion, thus rendering the image of an object more clear and distinct.