Appendix 37.

(938) [Sight relative to touch]

Condillac and Buffon said that objects seen upside down by the eye are put right by touch. It is extraordinary to see how this prejudice has been copied and repeated by one author after another. Hauy (Traité élémentaire de physique, vol. 2), Foderé (Physiologie positive, vol. 3) and Algarotti together with the whole band of our most recent authors have simply repeated the same thing. However, Melchiorre Gioia must be exempted. Despite numerous errors he made the following sound observation:

It seems absolutely false that sensations of touch can correct the impressions of sight. In fact, although touch assures us that a stick protruding from the water in a pond is straight, we see it bent and always see it bent, even if we touch it a thousand times. Again, although touch tells us that the image we see of ourselves in a mirror does not exist suspended in the air, our eye tells us that the image does exist, and we see it. An artist who has painted a sphere on canvas is certain that the sphere lies on a flat surface. His eye however tells him that a good part of the sphere emerges from the canvas towards him.
If we accept the explanation of physiologists that touch corrects the impressions of sight, objects should appear inverted until touch has removed the delusion. But this is not the case: people born with a cataract see objects the right way up, not inverted, when the cataract is removed.
Finally, animals that lack practically all touch should see objects inverted, but their behaviour makes us believe the opposite: they see objects the right way up, as we do.