OPTICS, BOOK I.

by the sight. And we find again that the colors of iron-colored bodies original: "corporum ferrei coloris" become brighter when a strong light shines upon them. We also find that when a strong light shines upon dense, white bodies, they appear to increase in whiteness and sparkle to our senses. Furthermore, we find that transparent bodies colored with deep hues—such as strong red wines kept in transparent vessels—appear black, dark, and almost opaque when they are in dim places or under weak lights; yet when they are in strong light, or when sunlight shines upon them, their colors brighten and their transparency becomes visible. Similarly, the colors of transparent colored stones appear muddy and dark when in dim places; but when a strong light shines on them, or they are placed facing the light—and if a white body is placed on the side opposite the light—their transparency becomes apparent due to the penetration of the light. Again, when transparent colored bodies are placed facing the light, and a white body is placed on the opposite side as we mentioned above: if the light is strong, the form of that color will appear in its shadow upon the white body opposite it; but if the light shining upon it is weak, only a shadow will appear on the opposite white body, and the color will not be seen. Additionally, we find that peacock feathers and the fabric called amilialmon original: "amilialmon, id est sericus uiridis mixtus cum fusco roseo." This refers to a type of iridescent "shot silk" that appears to change color based on the angle of light.—which is green silk mixed with a rosy brown—vary in color to the sight at different times of day, according to the variation of light shining upon them. These apparent conditions in colors therefore signify that the colors of objects are only perceived by the sight according to the lights shining upon them.

Since strong lights sometimes hide certain things within visible objects and sometimes reveal them to us, and weak lights sometimes reveal certain things and sometimes hide them; and since the colors of objects sometimes change according to the variation of light shining upon them; and since strong lights shining directly into the eye sometimes prevent the sight from perceiving certain objects; and yet, in all these cases, the sight perceives nothing of the visible world unless it is an illuminated form—it follows that what the sight perceives of a seen object depends entirely on the light within that object, the light shining upon the eye itself during the perception of that object, and the light within the air between the eye and the object. However, the reason why strong lights prevent the sight from perceiving certain visible things will be explained by us in our discourse on the nature of vision.

Chapter Four.###

The eye is composed of various membranes and bodies, and its origin and growth begin from the front part of the brain. From the anterior part grow two similar optic nerves original: "nerui optici," the nerves responsible for transmitting visual information., which begin to emerge from two locations on both sides of the front of the brain. It is said that each of these has two layers original: "tunicas" and that they grow from two membranes of the brain. They reach the middle of the outer and front part of the brain, where they meet and form a single "common" optic nerve. Then this nerve divides again into two equal and similar optic nerves. These two nerves then extend until they reach the two convexities of the two hollow eye-socket bones that contain the eyes. In the centers of these two hollow bones are two equally bored holes original: "foramina" or apertures. The nerves enter these two holes and emerge into the hollows of the two bones, where they dilate and expand. The end of each nerve becomes shaped like a funnel original: "instrumentum ponendi uinum in dolijs," literally a tool for putting wine into jars.. Each eye is built upon this extremity of the nerve—this funnel-like tool—and is joined firmly to it. The entire eye is composed of many layers.

The first of these is the white fat original: "pinguedo alba," referring here to the outer protective layer or sclera that fills the hollow of the bone; it is the largest part of the eye and is called the consolidativa The "consolidating" layer, known today as the sclera.. Inside this fat is a hollow round sphere, mostly black, though in some eyes it is green or gray-blue. The body of this sphere is thin, yet dense and not porous. Its exterior is attached to the consolidativa, and its interior is hollow. This layer is called the uvea From the Latin for "grape," because its dark, rounded shape resembles a grape skin.. In the middle of the front of the uvea is a round hole that passes through to its hollow center; this is positioned opposite the end of the hollow nerve upon which the eye is built. Covering this hole and the entire front of the uvea is a strong, white, transparent layer called the cornea So named because its texture and transparency resemble a thin, polished layer of horn..

Inside the hollow of the uvea is a small, white, moist sphere that receives the forms of visible things. Its transparency is not perfectly clear but has a certain thickness, resembling the transparency of ice; for this reason, it is called the glacialis The "icy" humor, now known as the crystalline lens.. It is positioned upon the end of the hollow nerve. In the front of this sphere is a slight surface compression, similar to the surface of a lentil Hence the modern term "lens.". The front surface is therefore a portion of a sphere larger than the spherical surface containing its two apertures...