CHAPTER II. Cylindrical Lenses

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.

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.”

After an examination with the Ski-optometer is completed, the total result of plus sphere and minus cylinder may be transposed if desired, though in most instances it is preferable to prescribe the exact findings indicated by the instrument. This will also avoid every possibility of error, eliminating responsibility where one is not familiar with transposition—since, after all, it is the duty of the optician to thoroughly understand that part of the work.

TRANSPOSITION OF LENSES

It is commonly understood that transposition of lenses is merely change of form, but not of value.

For example, a lens +1.00 sph. = -.50 cyl. axis 180° may be transposed to its equivalent, which is +.50 sph. = +.50 cyl. axis 90°. The accepted formula in this special instance is as follows: Algebraically add the two quantities for the new sphere, retain the power of the original cylinder, but change its sign and reverse its axis 90 degrees. Applying this rule, a lens +.75 sph. = -.25 cyl. axis 180°, is equivalent to +.50 sph. = +.25 cyl. axis 90°.

Similarly, a lens +1.00 sph. = -1.00 cyl. axis 180° is equivalent to +1.00 cyl. axis 90°.

One of the difficulties in transposing is in reversing the axis. In such cases, it is well to memorize the following simple rule:

To reverse the axis of any cylindrical lens containing three numerals—add the first two together and carry the last. For example, from 105 to 180 degrees, etc.:

105° Add—one and “0” equals 1 Then carry the 5 = 15° 120° Add—one and two equals 3 Then carry the 0 = 30° 130° Add—three and one equals 4 Then carry the 0 = 40° 150° Add—five and one equals 6 Then carry the 0 = 60° 165° Add—six and one equals 7 Then carry the 5 = 75° 180° Add—eight and one equals 9 Then carry the 0 = 90°

To transpose where there are but two numerals, 90° should be added.

In using the Ski-optometer, it is absolutely unnecessary to transpose the final result of an examination; merely write the prescription as instrument indicates. The idea that plus sphere combined with minus cylinder, or the reverse, is an incorrect method of writing a prescription, has long since been disproved.