Jackson: Decentering of Lenses for Prismatic Effects, with Glass having an Index of Refraction of about 1.54. On Testing Prismatic Lenses. The prescriber should be prepared to test the correctness of prismatic lenses when they are returned to him for inspection. A common usage is to select some object in the room with a long straight side to it such as the door jamb. With the spherocylindrical element of the lens properly neutralized the optical center of the whole combination is held before the observer's right eye and in line with the door jamb, when that portion of the a FIG. 57.—Effect of a prism in breaking the edge of a door jamb edge of the jamb seen through the lens will appear displaced to one side. (Fig. 57). The prism that will restore the displaced portion of the door jamb so that it appears continuous will be the measure of the prism that is incorporated in the lens. This same principle has been utilized by Zeigler in designing his card for measuring the prism strength of such lenses. After the spherocylindrical portion of the lens has been properly neutralized as in the preceding experiment, it is held at one or two meters from the card (depending on which distance the card is adapted for) and the prism strength immediately read off according to the amount of displacement produced. It is convenient, accurate and serviceable (Fig. 58). One other method remains, the use of the douziememeter.1 The instrument is applied to first the nasal and then the temporal edge of the lens, both of which should measure the same in case there is no prism element in the glass (Fig. 59). The same is true of the upper and lower edges of the lens. If the douziemeter registers say 10 douziemes for the nasal edge of a lens, and 6 douziemes for the temporal edge, the nasal edge being 4 douziemes thicker represents a 1 degree prism base in. A lens that is 2 douziemes thicker at the nasal than at the temporal edge would represent a 1/2 degree prism base in and a lens 8 1 The name is derived from the French douzieme-or one-twelfth-having reference to the old system of twelve lines to the inch. The scale of the douziemeter is marked in twelfths of an inch, or lines. douziemes thicker at the nasal than at the temporal edge would represent a 2 degree prism base in. The same method of measurement holds in measuring the upper and lower edges to detect any vertical prism that may be present in a lens. |
