The convergence index as a measure of the converging power / by Alexander Duane.

  • Alexander Duane
Date:
1914
Catalogue details

Licence: In copyright

Credit: The convergence index as a measure of the converging power / by Alexander Duane. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    9/12 (page 481)
    ]/2 interpupillary distance X ioo D and by C' the angle of convergence of the visual lines, then q/ = ci' + 30 if D is between 5 and 22cm. q/ = 0i' -f 20 if D is between 22 and 50cm. 0/ _ 0i' 4- x° if D is between 50 and ioocw. C = Ci' if D is over 100cm. Ci' in this case may be called the relative convergence index, inasmuch as it corresponds to a point upon which the eyes are converged when not using their maximum converging power. When the eyes are converging to the maximum, so that D represents the distance (PcB) of the real near-point of conver- gence, the convergence index may be called absolute. Since the convergence index by the simple relation indicated gives us the converging angle, it may be said to measure quite directly the power of convergence. It has been objected, however, that after all what we wish to know is, not so much the power as the availability of convergence. In other words, we wish to know, not how great the angle C is, but how close the near-point A is. The closeness of the latter, in fact, measures the usefulness of the converging power for the patient, just as the closeness of the near-point of accommodation measures the usefulness of the accommodation in enabling the patient to read. But there is a considerable difference be- tween the two cases. Two persons with the same range of accommodation may have near-points differing widely, inso- much that one of the two can read with the utmost ease and the other cannot read at all. But if one person has the same convergence index as another he cannot have an essentially different convergence near-point. In fact, for any ordinary value of the index, the variations in the position of the conver- gence near-point cannot possibly amount to more than 2cm,1 and are usually much less, so long as the index remains the same. Even when the index is low, and the near-point of 1 Thus, with a convergence index of 30 (quite the lower limit of normal converging power), the distance of the convergence near-point, i. e., the PcB, would be 9.0cm if the interpupillary distance was 54mm, and 11 .ocm if the interpupillary distance was 66mm.