A Simplified Subjective Method
The following simplified method of procedure is suggested for subjective testing with the Ski-optometer, although as previously explained, the refractionist may employ his customary method, overcoming the annoyance of transferring trial-case lenses and the setting of each cylinder individually. The Ski-optometer has been constructed and based upon the golden rule of refraction: “As much plus or as little minus spherical, combined with as little minus cylinder power as the patient accepts.”
By applying this rule as in the above method and starting with +5.D. spherical, watching the two zeros ([Fig. 4]) and rapidly reducing +1.25D each time, we will assume that +1.25D gives 20/30 vision; as a final result +1.D. will possibly give 20/25 vision.
The patient’s attention should next be directed to the most visible line of type, preferably concentrating on the letter “E” or the clock dial chart—either of which will assist in determining any possible astigmatism. Since the Ski-optometer contains concave cylinders exclusively, the next move should be the setting of its axis indicator at 180°, commonly understood as “with the rule.” One should then proceed to determine the cylinder lens strength by turning the reel containing the cylindrical lenses ([Fig. 8]). Should the patient’s vision fail to improve after the -.50D. cylinder axis 180° has been employed, the refractionist, in seeking an improvement, should then slowly move the axis indicator through its entire arc.
With the cylinder added, regardless of axis, poor vision might indicate the absence of astigmatism. If astigmatism exists, vision will usually show signs of improvement at some point, indicating the approximate axis. Once the latter is ascertained, the refractionist may readily turn the Ski-optometer’s cylinder reel and obtain the correct cylinder lens strength, after which the axis indicator should be moved in either direction in order to obtain the best possible vision for the patient.
The refractionist should always aim to obtain normal (or 20/20) vision with the weakest concave cylinder, combined with the strongest plus sphere, or weakest minus sphere.
Procedure for Using Minus Cylinders
Exclusively
For the benefit of those who have never used minus cylinders exclusively in making their examinations, we will assume that the patient requires O.U. +1.D sph. = -1D cyl. axis 180° for final correction; the latter, in its transposed form, being equivalent to +1.D. cylinder axis 90°. Unquestionably the best method is the one that requires the least number of lens changes to secure the final result.
To obtain this, the following order of lens change should be made: First, +1.D. sphere is finally determined and allowed to remain in place. Concave cylinders are then employed in quarters until the final results of +1.D. spherical, combined with -1.D. cylinder axis 180° is secured. This necessitates the change of but four cylindrical lenses as shown in routine “A” as follows:
| ROUTINE “A” | ROUTINE “B” | ||||||
| (Made with minus cylinder) | (Made with plus cylinder) | ||||||
| Sph. | +1.D. | Cyl. | Axis | Sph. | +1.D. | Cyl. | Axis |
| Step 1 | +1.D. = | -.25 ax. | 180° | equal to | +.75 = | +.25 ax. | 90° |
| Step 2 | +1.D. = | -.50 ax. | 180° | equal to | +.50 = | +.50 ax. | 90° |
| Step 3 | +1.D. = | -.75 ax. | 180° | equal to | +.25 = | +.75 ax. | 90° |
| Step 4 | +1.D. = | -1 ax. | 180° | equal to | 0 | +1 ax. | 90° |
In brief the method of using minus cylinders exclusively in an examination, as explained in routine “A”, necessitates the change of the cylinder lenses only after the strongest plus sphere is secured.
On the other hand, notwithstanding innumerable other methods where plus cylinders are used, routine “B” shows that the best spherical lens strength the patient will accept, is also first determined. Then both spheres and cylinders are changed in their regular order by gradually building up in routine, by increasing plus cylinder and next decreasing sphere, a quarter diopter each time, until the final result is secured.
While it is conceded that both routine “A” and “B” are of themselves simplified methods, by comparing routine “A” where minus cylinders are used with routine “B” where plus cylinders are used in their corresponding steps, the refractionist will note by comparison that one is the exact equivalent and transposition of the other. Where plus cylinders are employed, eight lens changes are made before final results are secured; while but four lens changes are necessary where minus cylinders are used.
The refractionist should also note by comparison that the use of minus cylinders reduces focus of the plus sphere, but only in the meridian of the axis. It has not made the patient myopic. Furthermore, a plus cylinder will bring the focal rays forward, while minus cylinders throw them backward toward the retina.
This is but another reason for the exclusive use of minus cylinders in refraction.
The method of using minus cylinders exclusively in an examination, necessitates the change of the cylinder lenses only. On the other hand, the method of using plus cylinders makes it necessary to change spheres and cylinders in routine.
In brief, since using the minus cylinder is merely a matter of mathematical optics, their use even in a trial-case examination is strongly urged.
The maximum value of the Ski-optometer is fully realized only when the advantages of using minus cylinders exclusively in every examination is clearly understood.