Concave Spherical Lenses
Another simple and exclusive Ski-optometer advantage worthy of note is the method employed in obtaining concave, spherical lenses. Instead of employing a battery of concave lenses similar to the convex battery previously described, the instrument’s operation is greatly simplified through the use of a neutralizing process.
In short, the Ski-optometer only contains two concave lenses to obtain its entire series—namely, a -6.D and a -12.D sphere ([Fig. 3c])—first setting the pointer of the supplementary disk at -6. sphere, then setting the indicator of the spherical battery at +6.
Thus zero (or plano) is obtained, the plus neutralizing the minus.
By merely turning the plus or convex spherical reel inward, or toward the patient’s nose, the convex power is then decreased, naturally increasing the concave value or total minus lens power. For example, if the spherical indicator shows +5.D, when the -6D. lens is placed behind it, the lens value at the sight opening will be -1D ([Fig. 6]). If required, the refractionist may continue on this plan until only the -6D. lens remains.
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.
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.