This large "condenser" lens is an accessory to microscope 116. The typical illumination setup is demonstrated here. Not shown is an oil lamp that would have been suspended on the articulated arm to the right of the large lens.
An alternate illumination system would have been an oil lamp such as this 19th Century one shown here. This lamp has an integrated condenser lens and back reflecting mirror. This system is more advanced than the condenser lens shown above, and was probably invented well after this Cuff-type microscope was manufactured.