“This instrument from the point to the head, near the handle (a to b, Fig. 36), is six-tenths of an inch, its cutting edge (a to c) is nearly four-tenths of an inch. The back is straight to near the point, where it is truncated so as to make the point stronger, but at the same time leaving it very acute, and the edge of this truncated portion of the back is made to cut. The remainder of the back is simply rounded off. The cutting edge is perfectly straight and is made to cut up to the part where the instrument becomes round, c. This portion requires to be carefully constructed, so that as the instrument enters the eye it shall fill up the incision, and thus prevent the escape of the aqueous humor.” [Illustration: Fig. 4.--Cheselden’s knife-needle (after Sharpe).] [Illustration: Fig. 37.--Ziegler’s model of knife-needle.] [Illustration: Fig. 36.--Hays’ knife-needle, exact size and enlarged (Hays).] [Illustration: Fig. 16.--Sickle-shaped knife, Aiguille-à-serpette (Galezowski).] [Illustration: Fig. 35.--Sichel’s iridotome (after Meyer).] [Illustration: Fig. 34.--Knapp’s knife-needle.] [Illustration: Fig. 7.--Adams’ iris-scalpel; large and small size.] [Illustration: Fig. 33.--Double edged lance-knife (modern model).] [Illustration: Fig. 5.--Double edged lance-knife (Beer).] [Illustration: Fig. 12.--Iris-knife (Walton, after Beer).] [Illustration: The Various Knife-Needles and Iris-Knives Mentioned in the Text. (Grouped together for study and comparison.)] [Illustration: Fig. 37.--Ziegler’s model of knife-needle.] 8. The knife-needle, which I invariably use, is a modified pattern of that devised by Hays. The form of this instrument lies midway between the falciform knife and the bistoury, and possesses the advantages of both. It has a very delicate point which punctures easily, and an excellent cutting edge of sufficient length (7 mm.). If the shank is properly rounded it can be used with a sawing motion, sliding backward and forward through the corneal puncture without injuring the cornea, and without allowing the aqueous to escape. To accomplish this the more easily, the shank has been made 4 mm. longer than the original model. This instrument, therefore, seems to meet all the requirements of a perfect iris-knife, viz., a falciform point which makes the best puncture, a straight edged blade which makes the best incision, and a cutting edge 7 mm. long, which is the best length for properly executing the sawing movement. My model[32] of knife-needle (Fig. 37) resembles Cheselden’s knife, as shown by Sharpe (Fig. 4), even more closely than the original pattern of Hays does. [32] This knife-needle has been carefully made for me by Luer, Paris, and by Ferguson, Philadelphia. ESSENTIALS OF SUCCESS IN IRIDOTOMY BY THE KNIFE-NEEDLE METHOD.