This experiment was believed to confirm all that Locke and Berkeley had foreseen, for it was long before the patient could distinguish objects by size, distance, or shape.[61] Condillac had renewed the interest which Voltaire had first kindled in the subject, by referring to Cheselden's experiment in his first work, which was published in 1746.[62] It happened that in 1748 Reaumur couched the eyes of a girl who had been born blind.

Diderot sought to be admitted to the operation, but the favour was denied him, and he expressed his resentment in terms which, as we shall see, cost him very dear.

As he could not witness the experiment, he began to meditate upon the subject, and the result was the _Letter on the Blind for the Use of those who See_.

published in 1749--the date, it may be observed in passing, of another very important work in the development of materialistic speculation, David Hartley's _Observations on man, his frame, his duty, and his expectations_.
Diderot's real disappointment at not being admitted to the operation was slight.

In a vigorous passage he shows the difficulties in the way of conducting such an experiment under the conditions necessary to make it conclusive.