Charles Darwin (1809-1882) published On the Origin of Species in 1859. It made public his theory of evolution, which was at once controversial and hugely influential. Between formulating the theory and publishing his findings Darwin worked on barnacles, and completed an entire classification of the species; this work involved large amounts of microscopy, and was related to his theory of evolution.
Charles Darwin was given his first microscope anonymously when he was an undergraduate at Christ's College, Cambridge. The donor later turned out to be a friend, John Maurice Herbert, and the gift was a good one: Darwin was enthralled by the possibilities that it offered.
One of the most important aspects of Darwin's intellectual development was the famous Beagle voyage, a circumnavigation of the globe. On December 27th 1831 he set out on board the Beagle. The voyage allowed Darwin to pursue his interests in geology, botany, anthropology, and biology. He took a small low-power microscope on the voyage (Image 1), and made many observations.
In 1846 Darwin started to sort through the some of the left over samples that he had collected on the voyage. Amongst these samples were some unusual barnacles, and, like many of his contemporary natural historians, he decided to work on the classification of a whole species. Darwin had become interested in invertebrates whilst at Edinburgh in the 1820s. He had already largely formulated his evolutionary theory before he set to work on the barnacles, and his theory informed his understanding of the species. However, the relationship between theory and experiment was complex. Darwin's study of barnacles was not only informed by his theory, but also helped to verify it. The barnacles demonstrated some of the traits Darwin had predicted: the loss of useless organs, and the transformation in function of organs which are structurally similar.
Many of the anatomical variations that divide the sub-species of cirripedia can only be observed under a high power microscope, and so, with the advice of his microscopist friends in mind, Darwin decided to buy a large compound microscope. He bought the microscope, which is now in the collections of the Whipple Museum, for £34 in 1847 (Image 2). In a letter of May 10th 1848 he wrote:
"I have purchased a 1/8" object glass, & it is grand. - I have been getting on well with my beloved cirripedia, & get more skilfull in dissection."
The 1/8" object glass was an impressive lens, even by modern standards. It allowed Darwin to achieve a maximum magnification of over 1300 times. The resolution and image quality at this magnification would have been poor, however, and from the measurements he made it can be estimated that Darwin carried out most of his high-power work at magnifications up to about 850 times.
Darwin had strict ideas about the use of the microscope. He thought that using high powers without using low powers was "injurous to natural philosophy". Whilst this does not necessarily mean that he lacked faith in higher power microscopes, it does show that he was aware of the mistakes that could be made by examining objects without the visual frame of reference provided by low powers.
This shows Darwin to be a skilled and sophisticated microscopist. In combination with his renowned talent for slide preparation, Darwin can be understood as a more practical man of science, using instruments and undertaking important experiments. He owned and used microscopes and was fascinated by both their working and what they revealed of the natural world.
Boris Jardine, 'Charles Darwin's microscopes', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2009 [http://www.hps.cam.ac.uk/whipple/explore/microscopes/darwinsmicroscopes/, accessed 26 November 2015]