The cataract is a collection of inert fluids upon the cornea at the pupil, obstructing vision, or preventing distinct vision. It arises most commonly from a congelation and weakness of the visual spirit, and on that account the disease rather attacks old persons, and those who are debilitated by protracted illness. It is occasioned also by violent vomiting, a blow, and many other causes. Those kinds of cataract which are but commencing, as not being proper objects of surgery, have been treated of in the Third Book. We shall now give the characters of those which are fairly formed and have acquired consistence. All those, therefore, who have cataract see the light more or less, and by this we distinguish cataract from amaurosis and glaucoma; for persons affected with these complaints do not perceive the light at all. Wherefore, again, Galen well instructs us as to the consistence and difference of cataracts and which kinds ought to be operated upon. Having shut the eye affected with the cataract, and with the large finger pressing the eyelid to the eye, and moving it with pressure to this side and that, then opening the eyelids and observing the cataract in the eye; if it has not yet acquired consistence, a certain flow takes place from the pressure of the finger, and at first it appears broader, but straightway resumes its former figure and magnitude. But in those which have acquired consistence no change takes place as to breadth or figure from the pressure. But since this appearance is common to those which are of moderate consistence, and those which are over-compacted, we distinguish these cases from one another by their colour. For those which are of an iron, cœrulean, or leaden colour, are of moderate consistence, and fit for couching; but those which resemble gypsum and hailstones are over-compacted. After ascertaining these circumstances, as directed by Galen, having placed the patient opposite the light, but not in the sun, we bind up carefully the sound eye, and having separated the lids of the other, at the distance from the part called the iris towards the small canthus, of about the size of the knob of the specillum, we then with the point of the perforator mark the place about to be perforated; and if it is the left eye we operate with the right hand, or if the right eye with the left; and turning round the point of the perforator, which is bent at its extremity, we push it strongly through the part which was marked out, until we come to an empty place. The depth of the perforation should be as great as the distance of the pupil from the iris. Wherefore, raising the perforator to the apex of the cataract, (for the copper of it is seen through the transparency of the cornea,) we push down the cataract to the parts below, and if it is immediately carried downwards, we rest for a little, but if it reascends we press it back again. After the depression of the cataract we turn round the perforator and extract it gently. After