Dr. Lancaster's statement that only those who know how to use the astigmatic dial can have satisfactory results with it is applicable also to other subjective methods of determining astigmatism. "Practice" alone is not sufficient. Any one desiring to work, for example, with the astigmatic dial or with the cross cylinder not only must be familiar with the optics of the procedure he chooses but must actually "see" the path of the rays, the focal lines and the interval of Sturm as they are being changed by it in the patient's eye. Only those who are really able to visualize such optical phenomena will be able to grasp and, after a few steps, actually foretell the patient's reactions to the measures undertaken in the course of an examination.
In his paper on the use of the cross cylinder, Pascal1 gave an exemplary demonstration of how such "visualizing" helps in
LINKSZ A. DETERMINATION OF AXIS AND AMOUNT OF ASTIGMATIC ERROR BY ROTATION OF TRIAL CYLINDER. Arch Ophthalmol. 1942;28(4):632–651. doi:10.1001/archopht.1942.00880100066005