from Duchamp, ed. Francis Naumann: "He even purchased a number of eye charts, one of which he sent to Walter Arensberg [above], hoping, as he said in an accompanying letter, that it might be of some use to him (presumably in his cyrptographic researches."
from Marcel Duchamp: artist of the century, ed. Rudolf Kuenzli and Francis M. Nauman (Cambridge: MIT Press, 1989), p. 211:
In a letter Duchamp sent from Buenos Aires to Louise Arensberg on January 7, 1919, he wrote: "I am also probably going to join the local chess club here to try my hand again. I had made up a set of rubber stamps (which I designed) with which I set up the games. I'm sending here an example for Walter. / By the same mail, Walter will receive an eye chart (with which oculists 'test' the eyes). Recently I have been using oculists' eye charts. And I hope that this one will be of use to Walter."
Unfortunately, the eye chart Duchamp sent to Arensberg has been lost. In a photograph taken a few years later in New York by Man Ray, however, of Duchamp's Rotary Glass Plates (Precision Optics), a number of eye charts with foreign letters and text appear attached to the wall of the artist's studio (see Jean-Hubert Martin, ed., May Ray: Photographs [New York: Thames and Hudson, 1982], plate 14).