CHAPTER II

CYLINDRICAL LENSES

It is commonly admitted that setting each trial-case cylindrical lens at a common axis is the most tedious part of refraction.

The automatic cylinder, one of the Ski-optometer’s latest and distinctly exclusive features, not only overcomes this annoyance but also avoids the need of individually transferring each cylindrical lens according to the varying strengths.

By merely setting the Ski-optometer’s axis indicator (Fig. 7), each cylindrical lens in the instrument automatically positions itself, so that it will appear at the opening at the _exact axis_ indicated.

This is readily accomplished by placing the thumb on the small knob, or handle of the axis indicator, drawing it outward so as to release it from spring tension. The indicator may then be set at any desired axis; and, on releasing the handle, every cylinder in the instrument becomes locked, making it _impossible_ for any lens to appear at an axis other than the one specified by the indicator.

This insures the _absolute accuracy_ of the axis of every cylinder as it appears before the patient’s eye. Subsequent shifting of the axis even to a single degree is impossible, although it is a common occurrence where trial-case lenses are employed.

OBTAINING CORRECT FOCUS

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.

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.