In this picture there are several optical toys which I use to teach students about Descartes’ philosophy. There is a kaleidoscope, a cube which appears to “float” inside of another cube, a telescope, a small tube which allows one to “peek sideways” (at a 90 degree angle). and strips of mirrored paper. Another optical toy which I have is a camera obscura.
In the First Meditation Descartes announces that he will proceed by trying to find some reason to doubt “the basic principles on which all my former beliefs rested” (AT VII, 18; Cottingham, Stroothoff and Murdoch, Vol. 2.). He immediately brings up the senses and how he has sometimes been deceived by objects which are very small or far away. He leaves the examples here unspecified, which leaves ample room for the readers to fill them in. In the Sixth Meditation, however, he does offer two specific examples. Descartes writes:
"Later on, however, I had many experiences which gradually undermined all the faith I had had in the senses. Sometimes towers which had looked round from a distance appeared square from close up; and enormous statues standing on their pediments did not seem large when observed from the ground” (VII. 76, CSM, V2).
These examples go by quickly, but from the Optics we know that Descartes had investigated both optics and catroptrics.(images produced by reflection such as from a mirror). Borel, one of Descartes’ first biographers, records how Descartes liked to “play” with optics. Borel writes:
“He [Descartes] was so eminent in the Mathematicks, that he did things beyond apprehension, espcially about prospectives; and to the end that he might prove his experiments, he prepared Prospectives of a large size, either of ice or of artificial black polisht marbel made hollow according to his desire, and the various forms he phancyed by the assistance of Brefsiaus, a most ingenious man, and when he had accomplished his design, he brake them into pieces and made new ones of the same matter. And before the same Brefsiaus, by a hidden secret in Optiks, he would show him a company of souldiers marching in his chamber, which he did much admire; but this proceeded from the small figure of souldiers which he conceal’d and brought forth multiplied without doors [this refers to a camera obscura effect, apparently].” (Pierre Borel, A summary or compendium of the life of the most famous philosopher Renatus Descartes, London. 1670).
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