THE OPTICS OF ALHAZEN

SON OF ALHAYZEN original: "Alhazen Filii Alhayzen." This is the Latinized name of Ibn al-Haytham (c. 965–1040), a pioneering scientist from Basra often called "the father of modern optics."

BOOK ONE.

Decorative woodcut initial 'P' featuring a scholar seated at a desk with an open book, surrounded by intricate floral and vine patterns.

THE FIRST Book is divided into seven chapters. The first is that light itself and illuminated colors perform some operation upon the sight. The second, that intense light hides certain visible things which weak light reveals, and vice versa. The third, that the colors of bodies appear different to the sight according to the difference of the lights shining upon them. The fourth is on the composition of the eye, its shape, and its position. The fifth declares the quality of vision and its dependence upon that composition. The sixth is on the function and utility of the instruments of sight. The seventh is on those things without which vision cannot be completed.

THAT LIGHT ITSELF, AND ILLUMINATED COLORS PERFORM SOME OPERATION UPON THE SIGHT. Chapter 1.

1. Light itself, and illuminated color strike the eyes. Witelo in hypothesis 6; proposition 16 of book 3. Witelo was a 13th-century Polish scholar whose work was heavily based on Alhazen; Risner published their works together in this 1572 edition.

We find that the sight, when it looks at very strong lights, suffers greatly from them and receives injury: for a viewer, when looking at the body of the sun, cannot look at it well because his sight suffers due to its light. And similarly, when he looks into a polished mirror upon which the light of the sun has fallen, and his sight is in the place to which the light is reflected from that mirror: he will suffer again because of the reflected light reaching 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 the viewer gazes at a pure white object upon which the light of the sun has fallen, and he dwells on the sight of it: then he turns his sight from it to a dark place of weak light: he can hardly perceive the visible things of that place with a true perception: and he will find a covering, as it were, between his sight and them: then little by little it is uncovered, and the sight returns to its normal state.

And again, when the viewer looks at a strong fire, and has gazed intently at it, and dwells on looking at it for a long time: then he turns his sight to a dark place of weak light: he will find the same thing again in his sight. And again we find, when a viewer looks at a pure white object upon which the light of day was shining—and that light was strong, even if it was not direct sunlight—and he dwells on the sight for a long time: then he turns his sight to a dark place: he will find the form In this context, "form" refers to the visual impression or image of the object. of that light in that place, and along with this he will find its shape: then if he closes his eyes, he will find in himself the form of that light: then this is taken away, and the eye returns to its normal state.

And the sight will be similarly disposed when it looks at an object upon which the light of the sun was shining. And similarly when it looks at a clearly white object upon which the light of a fire was shining, when the light of the fire is strong, and it dwells on looking at it: then it withdraws to a dark place: it will find this same thing again in its sight. And similarly when the viewer is in a house in which there is a large opening uncovered to the sky: and he looks from that place at the sky in the light of day, and dwells on looking at it: then his sight returns to a dark place in the house: he will find the form of the light which he perceived from the opening, with the shape of the opening, in the dark place: and if he closes his eye: he will find that form in it again. All these things, therefore, signify that light performs some operation upon the sight.

And we find again that when a viewer looks at a garden of very dense herbs original: "uiridarium multæ spissitudinis herbarum," likely meaning a lush green space or meadow. upon which the light of the sun was shining, and he dwells on looking at it: then he turns his sight to a dark place: he will find in that dark place a form colored by the greenness of those herbs: then if he looks at white visible things in that state, and those visible things are in shadow and a place of weak light: he will find those colors mixed with green: and if he closes his eye: he will find again in it the form of light and the form of greenness: then if he turns his sight to white visible things in a place of weak light: he will find those colors mixed with that color i.e., the green afterimage.. These things, therefore, signify that illuminated colors perform an operation upon the sight.

THAT INTENSE LIGHT HIDES CERTAIN VISIBLE THINGS which weak light reveals: and vice versa. Chapter 2.

2. Intense light obscures certain visible things which weak light illuminates: and vice versa. [See also propositions] 28, 97, 109, 150, 155, 156 of book 4.

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