This is made possible because the eye does not “accommodate” as quickly as the lens change made with the Ski-optometer. It should also be noted that the eye receives an image on its retina within one-sixteenth of a second; otherwise, the patient is forced to accommodate, making it difficult to see the difference of even a quarter diopter. On the other hand, in transferring trial-case lenses, with its slow, tedious procedure, the patient, being unable to detect the slight difference of only a quarter diopter, unhesitatingly replies, “no difference,” merely because they are compelled to accommodate.
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: