'In my dissections of animals' eyes at Grantham, I often marveled at their perfect sphericity, which, in bodies that were otherwise irregular grab-bags of bones, tubes, skeins and guts, seemed to mark them out as apart from all the other organs. As if the Creator had made those orbs in the very image of the heavenly spheres, signifying that one should receive light from the other,' Isaac mused aloud. 'Naturally, I wondered whether an eye that was not spherical would work as well. There are practical as well as theologic reasons for spherical eyes: one, so that they can swivel in their sockets.' There was some tension in his voice – the discomfort must have been appalling. Tears streamed down and spattered on the table like the exhaust from a water-clock – the only time Daniel ever saw Isaac weep. 'Another practical reason is simply that the eyeball is pressurized from within by the aqueous humour.'