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.
Chapter III
HOW THE SKI-OPTOMETER ASSISTS
IN REFRACTION
The construction of the Ski-optometer has now been fully explained, and the reader realizes that since the instrument contains all the lenses necessary in making an examination, greater operative facility is afforded through its use than where the trial-case lenses are employed.
The Ski-optometer is “an automatic trial-case” in the broadest sense of the term, wholly superseding the conventional trial-case. It should therefore be employed throughout an entire examination, wherever trial-case lenses were formerly used. To fully realize its labor saving value in obtaining accurate examination results, it is only necessary to recall the tedious method of individually handling and transferring each lens from the trial-case to the trial-frame, watching the stamped number on each lens handle, wiping each lens and in the case of cylindrical lenses setting each one at a designated axis—all being needless steps where the Ski-optometer is employed.
The Use of the Ski-optometer in Skioscopy
In skioscopy, the Ski-optometer offers the refractionist assistance of the most valuable character.
For example, assuming that extreme motion in the opposite direction with plane or concave mirror is obtained with a +1.25D. spherical lens before the patient’s eye; by quickly turning the Ski-optometer’s single reel until the two white zeros again appear, +2.50D is secured, as explained in the previous chapter. If this continues to give too much “against motion,” the lens power should be quickly increased to +3.75 or +5.00D if necessary ([Fig. 4]). Should the latter reveal a shadow in the reversed direction, the refractionist is assured that it is the weakest lens that will cause its neutralization. Practically but few lenses have been used to obtain the final result proving the instrument’s importance and time-saving value in skioscopy, and demonstrating the simplicity with which tedious transference of trial-case lenses is avoided.