At the session of the American Medical Association in 1912, I had the honor of presenting a paper which dealt especially with the measurement of fatigue of convergence and of divergence as registered by the ergograph. In another paper in 1913, at the meeting of the American Ophthalmological Society, a classification was given of the varieties of fatigue of convergence. In this third paper I show that by a similar method the ciliary muscle can also be made to write its story of fatigue. The principle involved has now been worked out sufficiently to venture a description of the method, but the subject has proved to be so vast and apparently so important clinically, that the fatigue of accommodation which accompanies forms of ametropia or of heterophoria must be reserved for future consideration. It will add to clearness to give first a simple illustration of what is meant by the