The Errors of Sight

Explanation of the particular errors of sight, to serve as an example of the general errors of our senses.

I have explained how to recognize the errors of our senses regarding sensible qualities in general, which were discussed on the occasion of light and colors, as the order of things required that they be explained.

It would seem that we should now descend somewhat into particulars and examine in detail the errors into which each of our senses leads us; but we shall not dwell on these things, because after what has already been said, a little attention will easily suffice in place of the tedious discourses one would otherwise be obliged to deliver. We shall only report the general errors into which our sight causes us to fall concerning light and colors, and it is believed that this example will suffice to make us recognize the errors of all the other senses.

When we have looked at the sun for a few moments, here is what happens in our eyes and in our soul, and the errors into which we fall.

Those who know the first elements of dioptrics and something of the admirable structure of the eyes are not unaware that the rays of the sun undergo refraction in the crystalline lens and in the other humors, and that they then gather on the retina or optic nerve, which lines the entire back of the eye, in the same way that the rays of the sun passing through a magnifying glass or convex lens gather at the focus or burning point of that lens, two, three, or four inches from it, in proportion to its convexity. Now, experience teaches that if one places at the focus of this lens some small piece of cloth or black paper, the rays of the sun make such a strong impression on this cloth or paper, and agitate its small parts with such violence, that they break them and separate them from one another; in short, they burn them or reduce them to smoke and ashes [25].

Thus, one must conclude from this experience that if the optic nerve were black, and if the pupil or the hole of the iris through which light enters the eyes widened to let the sun’s rays pass freely, instead of contracting to prevent them, the same thing would happen to our retina as to that cloth or black paper, and its fibers would be so violently agitated that they would soon be broken and burned. It is for this reason that most men feel great pain if they look at the sun for a moment, because they cannot close the hole of the pupil so well that enough rays do not still pass through to agitate the filaments of the optic nerve with much violence and with some cause to fear that they may break.

The soul has no knowledge of all that we have just said; and when it looks at the sun, it perceives neither its optic nerve nor that there is motion in this nerve; but this is not an error, it is only simple ignorance. The first error into which it falls is judging that the pain it feels is in its eye.

If, immediately after having looked at the sun, one enters a very dark place with eyes open, this shaking of the fibers of the optic nerve caused by the sun’s rays diminishes and gradually changes. This is all the change that one can conceive in the eyes. However, this is not what the soul perceives, but only a white and yellow light; and the second error is that it judges that the light it sees is in its eyes or on a wall near us.

Finally, the agitation of the fibers of the retina always diminishes and gradually ceases; for when a body has been shaken, one ought to conceive nothing else in it than a diminution of its motion; but this is still not what the soul feels in its eyes. It sees that the white color becomes orange, then changes to red, and finally to blue. And the third error into which we fall is that we judge that there are in our eye or on a wall near us changes that differ much more than merely in degree (more or less), because the blue, orange, and red colors that we see differ among themselves in a manner quite other than by more or less.

Here are some errors into which we fall concerning light and colors; and these errors lead us into many others, as we shall explain in the following chapters.