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.
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.