Vision and the dimensions of nerve fibers.

Vision provided the obvious source of determining the dimensions of nerve fibers when suitable achromatic microscopes were directed at neural tissue in the 1830s. The earlier microscopes of Hooke and Leeuwenhoek were unable to resolve such small structures adequately. However, it was not Hooke's microscope that led to an estimate of the dimensions of nerve fibers, but his experiments on the limits of visual resolution; he determined that a separation of one minute of arc was the minimum that could normally be seen. Descartes had earlier speculated that the retina consisted of the ends of fibers of the optic nerve, and that their size defined the limits of what could be seen. Estimates of the diameters of nerve fibers were made on the basis of human visual acuity by Porterfield in 1738; he calculated the diameters of nerve fibers in the retina as one 7,200th part of an inch (0.0035 mm), based on the resolution of one minute as the minimum visible. In the same year, Jurin questioned the reliability of such estimates because of variations in visual resolution with different stimuli.