From around 1709 Jacques Cassini gradually took over his father's duties as head of the Paris Observatory. Cassini's health began to deteriorate, in particular his eyesight became poor so that by 1711 he was nearly completely blind. Fontenelle says that Cassini's calm and gentle character, coming from a deeply religious belief, allowed him to bear nearly total blindness with good cheer.
Mula sa humigit-kumulang na 1,709 Jacques Cassini unti-unti kinuha sa kanyang ama tungkulin bilang pinuno ng Paris obserbatoryo. Cassini's kalusugan ay nagsimulang lumubha, partikular na sa kanyang paningin ay naging mahihirap na kaya siya sa pamamagitan ng 1711 ay halos ganap na bulag. Fontenelle nagsasabing na Cassini's kalmado at banayad na character, na nagmula sa isang mataimtim relihiyosong paniniwala, pinahihintulutan sa kanya upang madala halos buong kabulagan sa mabuting magsaya.