A Simplified Skioscopic Method
In using the Ski-optometer, instead of working forty inches away from the patient in skioscopy and deducting 1.D., the refractionist will find it more convenient to work at a twenty inch distance, deducting 2.D. This working distance may be accurately measured and maintained by using the reading rod accompanying the instrument. Instead of deducting 2.D. from the total findings, however, it is preferable to insert a +2.D. trial-case lens in the rear cell of the instrument directly next to the patient’s eye. After determining the weakest lens required to neutralize the shadow in both meridians, the additional +2.D. lens should be removed and the total result of the examination read from the instrument’s register.
To illustrate a case in skioscopy where spherical lenses are employed to correct both meridians, assume that the vertical shadow requires a +1.25D lens to cause its reversal, while the horizontal requires +2.00D. Employment of the customary diagram, illustrated in [Fig. 10], would show the patient required +1.25 sph. = +.75 cyl. axis 90°, which when transposed is equivalent to +2.00 sph. = -.75 cyl. axis 180°.
Fig. 10—Where spherical lenses are employed in skioscopy, above indicates patient requires
+1.25 Sph. = +.75 Cyl. Axis 90°
or +2 Sph. = -.75 Cyl. Axis 180°
It should be noted that the total spherical power is +2.00D, as the Ski-optometer’s register shows, while the difference between the two meridians is 75, which is the required strength of the cylinder. By then turning the cylinder reel to .75, and setting the axis indicator at 180° (because by using minus cylinders, the axis must be reversed) the patient should read the test-type with ease if the skioscopic findings are correct. Thus with the Ski-optometer, it is not even necessary to learn transposition, since the instrument automatically accomplishes the work, avoiding all possibility of error.
Employing Spheres and Cylinders
in Skioscopy
Another commonly used objective method may be employed with even greater facility through the combined use of both the Ski-optometer’s spherical and cylindrical lenses. As previously suggested, insert the +2.00 spherical trial-case lens in the rear of the instrument, working at a twenty inch distance, then proceed to correct the strongest meridian first.
It was assumed that it required a +2.00 spherical to neutralize the strongest, or horizontal meridian, as shown in [Fig. 10]. The refractionist should then set the axis indicator therewith, which is the axis of the cylinder, or 180°.
It is then merely a matter of increasing the Ski-optometer’s cylindrical lens power until the reversal of the shadow in the weakest meridian is determined. Assuming this proves to be -.75 cylinder, axis 180°, the patient’s complete prescription +2.00 sph. = -.75 cyl. axis 180°, would be registered in the Ski-optometer without any further lens change other than the removal of the +2.00 working distance lens.
However, regardless of the method employed, the Ski-optometer greatly simplifies skioscopy. In fact, the instrument was originally intended to simplify retinoscopy or skioscopy, as the subject should be termed, the name “Ski-optometer” having been derived from the latter.
Use of the Ski-optometer
in Subjective Testing
In subjective refraction, especially where the “better or worse” query must be decided by the patient, it is commonly understood that the refractionist is compelled to first increase and then decrease a quarter of a diopter before the final lens is decided. With the Ski-optometer, the usual three final changes are made in far less time than it takes to make even one lens change from trial-case to trial-frame.
For example:
Assuming, with a +1.25D spherical lens before the patient’s right eye, he remarks that he “sees better” with a +1.D. while +.75D is not as satisfactory. The refractionist can then quickly return to +1.D., simply turning the Ski-optometer’s single reel outward to increase, or backward to decrease, the lens strength. So rapidly have these lens changes been made, that the patient quickly sees the difference of even a quarter diopter, and quickly replies, “better” or “worse.”
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.