OF ALHAZEN, SON OF ALHAYZEN

OPTICS

BOOK ONE.

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The first book is divided into seven chapters. The first is that light by itself, and illuminated colors, produce some effect upon the vision. The second is that intense light hides certain visible things that weak light reveals, and conversely. The third is that the colors of bodies are diversified to the sight according to the diversity of lights rising upon them. The fourth is concerning the composition, form, and position of the eye. The fifth declares the quality of vision and its dependency upon it. The sixth is concerning the office and utility of the instruments of vision. The seventh is concerning those things without which vision cannot be completed.

1. Light by itself and illuminated color strike the eyes. Vitellio in hypotheseis 6. 16 p 3.

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We find that sight, when it looks upon very strong lights, will ache strongly from them and will suffer harm: for when the viewer looks upon the body of the sun, he cannot look at it well, because his sight will ache on account of its light. And similarly, when he looks upon a polished mirror upon which the light of the sun was ascending, and his sight is in the place to which the light is reflected from that mirror, it will ache again on account of the reflected light arriving at his sight from the mirror, and he will not be able to open his eye to look at that light. And we find again, when a viewer gazes upon a clean white body upon which the light of the sun was ascending, and lingers in his gaze upon it, and then turns his sight from it to a dark place of weak light, that he will hardly be able to comprehend the visible things of that place with true comprehension; and he will find a covering as if between his sight and them; then, little by little, it will be uncovered, and the sight will return to its state. And again, when a viewer looks upon a strong fire and has gazed upon it and lingers in looking for a long time, and then turns his sight to a dark place of weak light, he will find the same again in his sight. And again we find, when a viewer looks upon a clean white body upon which the light of day was rising—and that light is strong, even if it is not the light of the sun—and lingers in his gaze for a long time, and then turns his sight to a dark place, he will find the form of that light in that place and will find with this its shape; then, if he closes his sight, he will find in it the form of that light; then this will be taken away, and the eye will return to its state. And the sight will be disposed in the same way when it has looked upon a body upon which the light of the sun was rising. And similarly, when it has looked upon a clearly white body upon which the light of fire was rising, when the light of the fire is strong, and he lingers in looking upon it, and then retreats to a dark place, he will find this same thing again in his sight. And similarly, when a viewer is in a house in which there is a large hole opened to the sky, and he looks from that place at the sky in the light of day, and lingers in looking at it, and then his sight returns to a dark place in the house, he will find the form of the light that he was comprehending from the hole, along with the shape of the hole in the dark place; and if he closes his eye, he will find that form in it. All these things, therefore, signify that light produces some effect upon the vision. And we find again that when a viewer looks upon a garden with a great thickness of herbs upon which the light of the sun was rising, and lingers in looking upon it, and then turns his sight to a dark place, he will find in that dark place a colored form from the greenness of those herbs; then, if he looks at white visible things in that state—and those visible things are in the shade and in a place of weak light—he will find these colors mixed with the greenness; and if he closes his eye, he will find in it again the form of the light and the form of the greenness; then that will be uncovered and taken away. And similarly, if he looks upon a body colored with a blue or red color, or another strong, sparkling color upon which the light of the sun was rising, and lingers in looking upon it, and then takes his sight to white visible things in a place of weak light, he will find those colors mixed with that color. These things, therefore, signify that illuminated colors produce an effect upon the vision.

2. Intense light obscures certain visible things which weak light illuminates: and conversely. 28. 97. 109. 150. 155. 156 p 4.

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And again, we see the stars in the night, and we do not see them in the light of day; and there is no difference between the times except that the air between our sight and the sky is illuminated by day, and by night...