This paper claims that the testing of microscopes during the eighteenth century reveals specific types of interaction between makers and users and links between scientific and economic interests. Basic procedures for the comparison and test of microscopes existed already in the Enlightenment although many historians thought that these were invented during the nineteenth century. The paper discusses three kinds of tests, advertising, the admission of a microscope in the laboratory, and finally the selection test. The advertising test deals with the use of measures to advertise microscopes by makers and reveals the importance and variety of a growing competing market in Europe. Second, the admission test regards the procedure a scholar carried out in his lab when he received a new microscope. This test concerns the standardisation of scientific instruments for the reproduction of experiments in the lab. Thirdly, scholars, and not makers, performed selection tests to choose the best microscope among several, showing the interconnection between market and scientific interests. These three tests illustrate three kinds of relationships between the producers and consumers of these instruments: 1. shaping users, 2. converting makers' products into scientific devices, and 3. rescuing makers.