After setting the axis indicator, the only remaining move is to obtain the correct cylindrical strength or focus. This is readily accomplished by merely turning the Ski-optometer’s larger or extreme outer single reel, which contains concave cylindrical lenses from .25D to 2D in quarters (Fig. 8a). It should again be borne in mind that a downward turn _increases_ concave cylinder power, while an upward turn _decreases_ it. The operation of the cylinder reel is greatly facilitated by carefully noting position of thumb and index finger (Fig. 8). Thus accuracy of result, simplicity of operation and the saving of much valuable time is invariably assured. [Illustration: Fig. 8A—Inner cog-wheel construction, showing arrangement of Ski-optometer cylinders. This simple construction assures accuracy and avoidance of the slightest shifting of axes.] As each cylinder appears before the patient’s eye, it simultaneously registers its focus at the indicator marked “CC CYL” shown in Fig. 8. Examinations of greater accuracy could not possibly be made than those obtained through the Ski-optometer, hence no refractionist should hesitate to employ it throughout an _entire_ examination—wherever trial-case lenses are used. The range of the Ski-optometer’s cylinder lens battery includes up to 2D. in quarters. An axis scale and a cell is located at the back of the instrument for insertion of an additional trial-case cylinder lens, when stronger cylindrical power is required. For example, if an additional -2D. cylinder is added, it will increase the range up to 4D. cylinder; or if twelfths are desired, a 0.12D. cylinder lens may be inserted. In this connection, it is interesting to note that considerable experimenting with _twelfths_ in the Ski-optometer proved them to be needless, inasmuch as the instrument’s cylindrical lenses set directly next to the patient’s eyes overcome all possible loss of refraction, as explained in a later paragraph. [Illustration: Fig. 8—Turn this Single Reel as shown by dotted finger to obtain cylindrical lenses, which simultaneously register their focus as they appear. Each lens also automatically positions itself at axis designated]. WHY CONCAVE CYLINDERS ARE USED EXCLUSIVELY The Ski-optometer contains only concave cylinders, as it is universally admitted that convex cylinders are not essential for testing purposes. In fact, concave cylinders should alone be used in making an examination, even where a complete trial-case is employed. To repeat one of the first rules of refraction: “As _much_ plus or as _little_ minus spherical power as patients will accept, combined with weakest minus cylinder, simplifies the work of refraction and insures accuracy without time-waste.”