Here, as in the other part of his essay, Diderot followed the method of interrogating the blind themselves. In this instance, he turned to the most extraordinary example in history, of intellectual mastery and scientific penetration in one who practically belonged to the class of the born-blind; and this too in dealing with subjects where sight might be thought most indispensable. From 1711 to 1739 one of the professors of mathematics at Cambridge was Nicholas Saunderson, who had lost his sight before he was twelve months old. He was a man of striking mental vigour, an original and efficient teacher, and the author of a book upon algebra which was considered meritorious in its day. His knowledge of optics was highly remarkable. He had distinct ideas of perspective, of the projections of the sphere, and of the forms assumed by plane or solid figures in certain positions. For performing computations he devised a machine of great ingenuity, which also served the purpose, with certain modifications, of representing geometrical diagrams. In religion he was a sceptic or something more, and in his last hours Diderot supposes him to have engaged in a discussion with a minister of religion, upon the arguments for the existence of a deity drawn from final causes. This discussion Diderot professes to reproduce, and he makes Saunderson discourse with much eloquence and some pathos. By one of those mystifications which make the French polemical literature of the eighteenth century the despair of bibliographers, Diderot cites as his authority a _Life of Saunderson_, by Dr. Inchlif. He sets forth the title with great circumstantiality, but no such book exists or ever did exist. The Royal Society of London, however, took the jest of fathering atheism on one of its members in bad part, and Diderot was systematically excluded from the honour of admission to that learned body, as he was excluded all his life from the French Academy.