After an examination with the Ski-optometer is completed, the total result of plus sphere and minus cylinder may be transposed if desired, though in most instances it is preferable to prescribe the exact findings indicated by the instrument. This will also avoid every possibility of error, eliminating responsibility where one is not familiar with transposition—since, after all, it is the duty of the optician to thoroughly understand that part of the work. 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.