The Greater Work (Opus Majus), Vol. 2

portions of different spheres, and therefore when these spheres intersect, it is necessary that they have different centres. And since the concavity of the vitreous is toward the crystalline, then its centre is beyond the centre of the eye toward the anterior of the eye. Similarly, the centre of the anterior crystalline is in the depth of the eye. Yet these centres are upon the same straight line which enters through the hole of the anterior uvea and through the hole which is at the extremity of the nerve, where the retina begins to expand. For these bodies are thus ordered, according to the authors of Perspective, namely that from the hole of the bone, where the nerve enters, the nerve extends for some space, and is always more dilated, until it comes to the circumference of the sphere of the entire crystalline, and is consolidated with its circumference. And then upon the extremity of the nerve the whole crystalline is composed, and is contained in the lower part of the uvea, which Alhazen calls the point of the concavity of the uvea, in the last part of which is the hole which is the extremity of the nerve, where the uvea begins. But the middle of the whole crystalline, namely the vitreous humour, is in the orifice of the hole. For the extremity of the nerve contains the middle of the sphere of the entire crystalline, as Alhazen says, which middle is the vitreous humour; and the uvea is consolidated with the circumference of the sphere of the crystalline. And the aqueous humour contained in the uvea touches the sphere of the anterior crystalline, and that fills the hole up to the contact of the cornea, not that it touches the cornea at a point, but through the application of surfaces, just as an interior sphere is contained by an exterior one. Since, however, the convex surface of the cornea is continued with the surface of the whole eye, and with the whole eye, as Alhazen says, it is necessary that they have the same centre. And since the concave surface of the cornea is equidistant to the exterior convex surface, it is necessary that both surfaces of the cornea and the whole eye have the same centre, by the book of Theodosius Theodosius of Bithynia, an ancient Greek mathematician, author of Sphaerics. on spheres: for spheres equidistant to each other, one containing and the other contained, have the same centre, like the spheres of the world, such as the starry heaven, and the sphere of fire, similarly in others: for the centre of the world is the centre of all things. And since the concave surface of the cornea, and the convex surface of the aqueous humour, which is in the hole, are like an interior and exterior sphere, it is necessary that the convex surface of the aqueous humour have the same centre as those aforesaid. But because the concave surface