We are recommended to perform it in two sets of cases:-- 1. Those in which the eye is known to be unhealthy and liable to inflammations, specially of iris, retina, or choroid. In cases where the patient has already lost an eye, Von Graefe thinks iridectomy should always precede extraction. In the above, then, it is a precautionary measure, and, if convenient, should be performed three, four, or even six weeks before the extraction. 2. It is recommended to be performed at the same time as extraction in all cases in which the operation has presented any special difficulties, or has not gone smoothly, _e.g._ in cases where the lens has required much force to expel it, either from the flap of cornea being too small, or from adhesions between the lens and capsule; or, again, in cases in which there is a tendency to prolapse of the iris, in which any of the cortical substance has been necessarily left behind, or in which old adhesions had existed between the iris and capsule, or between the cornea and iris. OPERATIONS FOR ARTIFICIAL PUPIL.--The cases are by no means unfrequent in which it is necessary to remove or destroy a portion of the iris to admit light to the retina. In cases of excessive prolapse of the iris after extraction of the lens, where the iris has formed adhesions to the wound, and still more frequently in cases where central opacities of the cornea have fairly occluded the natural pupil, the only chance for vision is to enlarge the old one, or make a new pupil by removal of the iris. Very various operations have been proposed, and exceedingly numerous and complicated instruments invented for this purpose. We can notice here only one or two of the most approved procedures:-- 1. _Incision_ is the simplest. This is practicable and effectual only in cases where the iris is so far healthy as still to retain its contractile power, and so far free from adhesions as to be able to make use of it. The best example of such a case is that of a cataract, in which after extraction a prolapse of the iris has occurred to such an extent as to obliterate the pupil, and where, at the same time, the only adhesions are to the wound, none to the cornea. _Operation._--A double-edged needle is introduced through the cornea near its margin; on arriving at the place where the pupil ought to be, one edge is drawn against the iris, and divides it transversely, if possible, without injuring the lens; the fibres of the iris start back, contract, so that a sufficiently large central pupil may be obtained. 2. _Excision._--In the far more frequent cases in which there exist adhesions between iris and cornea, or iris and anterior capsule, incision is not sufficient, and it is necessary to excise a portion of the iris. The simplest and safest operation is the following:--