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
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.”
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.