The mechanism of accommodation has been the subject of much study and of much controversy. There is no agreement yet as to the exact mechanism of accommodation. There is evidence, however, that accommodation is subject to fatigue, i. e., decrease of power to accommodate resulting from prolonged exercise. Some authorities regard fatigue of the accommodative neuromuscular system as the central factor in the mechanism of ocular fatigue.
Positive results with respect to fatigue of accommodation have been reported by a number of investigators. Berens and Stark1 reviewed the literature up to 1932. They discussed the positive findings of Ferree,2 Lancaster and Williams3 and Howe4 and the work of Berens and his associates from 1918 on.5
Ferree,2 using a method which consisted in observing the letters "li" on a white card set at a distance within the punctum remotum for a period of three minutes,