A woman dropping her porcelain tea-cup in horror upon discovering the monstrous contents of a magnified drop of Thames water; revealing the impurity of London drinking water. Coloured etching by W. Heath, 1828.

The caricature shows a woman in fashionable dress looking into a microscope to observe little monsters swimming about in a drop of London Thames water. In the 1820s much of the drinking water of Londoners came from the river Thames, and the sewers emptied into the Thames. A Commission on the London Water Supply was appointed to investigate this dangerous situation, and it reported in 1828. After that report, the five water companies which served the north bank of the river improved their supplies by building reservoirs etc., but the people of Southwark (on the south bank of the river) continued to receive infected water. The problems were not solved until the 1860s when London's present sewerage system was installed by the Metropolitan Board of Works (MBW) and its engineer Joseph Bazalgette. Between the date of this caricature (1828) and the completion of the MBW sewers, London suffered two cholera epidemics, one in 1832 (part of the world pandemic of cholera) and one in 1854. Looking at a drop of water though a microscope was a popular entertainment provided by travelling showmen who carried the microscopes around in cases on their backs