Mathematical Mirror

it is called the cornea horny layer, and it is composed, as Avicenna says in the mentioned book, of four subtle cortical tunics, and they are like barks, so named, so that if one of them is peeled off, the others are not harmed because of this. And it is made so, that this tunic may be strong against lesions and external impressions coming from the air, and it is very transparent, so that the multitude of its tunics does not impede the passage of the images of visible things. The third tunic of the eye is made from that third film of the nerve, which comes from the membrane of the cornea, and its first part is joined to the bone of the eye, and it is so hard and solid that it is therefore called the sclerotica hard/white of the eye; the remaining part, however, is extended to the cornea. For this tunic is not complete, but a portion of the sphere is missing, and it is filled with fatty white flesh, as we see externally in eyes, and it is called the consolidative or conjunctiva. It must be considered diligently, however, that in one way there are said to be three tunics only, and in another way six: and both considerations are true and reasonable. For if we consider the whole tunics, there are only three. If, however, we consider the posterior parts distinct from the anterior ones in both name and reality, in this way there are six. For three parts are from the posterior and three from the anterior. Some also wanted there to be fewer, and this by a multiple consideration, but one should not care about these, because their interpretation is wicked and deviates from right reason. Some also posited seven tunics. But it is false, because they counted the spider's web the retina as a tunic, when it is not. And those who say there are three, call the whole first tunic the uvea, and the whole second the cornea, and the whole third the consolidativa. Hence the authors of the Perspectiva call the whole first one the uvea, and thus I wish to use it principally in executing the mode of seeing. And therefore Alharen says that the uvea has two holes: one anterior and the other posterior, which is the hole of the nerve from which the expansion of the concavity of the uvea begins, where the extremity of the nerve with the whole following concavity up to the anterior hole is the uvea according to the truth.