[Illustration: Fig. 6—With this indicator of supplementary disk, set at -6.D. Sph. and spherical indicator at +5.D—lens value at sight opening is -1.D. Sph. This simple arrangement makes it possible to operate the Ski-optometer with but Single Reel for both plus and minus sphericals.] Should concave power stronger than -6D. be desired, by placing the pointer of the supplementary disk at -12D. Sph. and proceeding to neutralize as before, all the concave powers up to -12D. in quarters are similarly obtained. For the convenience of the operator, all minus or concave spherical powers are indicated in _red_; while plus, or convex powers, are indicated in _white_. The instrument is also provided with an opaque or blank disk which is brought into position before the sight opening by setting the pointer of the supplementary disk at “shut” (Fig. 3c.) Summing up, all plus and minus spherical powers have been attained from zero to 12D. in quarters, practically through the turn of the single reel—_a simplicity of operation largely responsible for Ski-optometer supremacy_. 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. [Illustration: Fig. 7—Once you set the axis indicator as shown by dotted fingers, each cylindrical lens in the instrument automatically positions itself exactly at that axis, as indicated by the arrow.] 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