During the last years of the 17th century many people speculated about the future and importance of the microscope. Some thought that it had shown all it could, and some recent historians have accepted this view. In spite of this, some important discoveries were still being made, such as the existence of capillaries. Microscopes sold in the early 18th century often included a 'frog-plate' or 'fish-plate', so that anyone could view these tiny blood vessels.
The circulation of blood was only fully understood during the 17th century. William Harvey (1578-1657) proposed the circulation of blood, based on his dissection work on the heart. Whilst Harvey's theory became generally accepted, it had one major flaw: he could not account for the movement of blood between arteries and veins, and for his theory of circulation to make sense this movement was necessary. Circulation was only fully understood after Harvey's death, when Marcelo Malpighi and Antoni van Leeuwenhoek observed capillaries, tiny blood vessels that carry the blood between arteries and veins.
A microscope capable of magnifying roughly 100 times is capable of showing capillaries, and at the beginning of the 18th century these microscopes were becoming available for interested and educated members of the public. Microscopes were often sold with a selection of prepared slides, and also with a 'frog-plate' or 'fish-plate'. The Whipple Museum has three such accessories, which allow a fish or a frog to be strapped down for observation. Image 1 shows a brass frog-plate by George Adams, and Image 2 shows a glass example.
Once the frog or fish tied down, capillaries can be seen in the leg or fin as long as a sufficiently strong light is shone through the animal.
It remains unclear how widely these accessories were used. They would have been less convenient to use than the slides that were sold. To use the frog-plate an animal would have to be caught and then tied down live or recently-deceased. Most frog-plates held in museums demonstrate no obvious evidence of use. However, the frog-plate remained a selling point for the microscope. Public demonstrations of microscopes became increasingly popular during the 18th century, and frog-plates may well have been used at these organised events rather than by interested amateurs at home.
Boris Jardine, 'Fish-plates and frog-plates: live examination under the microscope', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2008 [http://www.hps.cam.ac.uk/whipple/explore/microscopes/fishandfrogs/, accessed 23 March 2017]