During the 19th century there was no clear boundary line between those who were considered to be part of the scientific community and those who were seen as outsiders. It was during this century that the categories of "professional scientist", "amateur" and "popularizer of science" were being debated and constructed. As a result, in recent times scholars of the period have explored the important roles of neglected amateurs, women and members of the working class. Scholars in the field are continually broadening their definition of the terms "science" and "scientist". This dictionary contains more than 1200 entries on both major and minor figures who had an impact on British science. By examining how the theories and practices of scientists were shaped by Victorian beliefs about religion, gender, imperialism and politics, the dictionary presents a rich panorama of the development of science in the 19th century. As well as containing entries on those working in traditional scientific areas, such as geology, physics, astronomy, chemistry, mathematics and biology, the dictionary also covers the human sciences such as anthropology, sociology, psychology and medicine. In addition, areas such as phrenology, mesmerism, spiritualism, scientific illustration, scientific journalism and publishing, instrument making and government policy are covered. By including new figures working in these areas, and by paying attention to the social and cultural context in which they lived, the dictionary reflects the richer picture of the 19th-century period gradually being developed by scholars in the field.