Fig. 8—Turn this Single Reel as shown by dotted finger to obtain cylindrical lenses, which simultaneously register their focus as they appear. Each lens also automatically positions itself at axis designated.
Why Concave Cylinders Are Used Exclusively
The Ski-optometer contains only concave cylinders, as it is universally admitted that convex cylinders are not essential for testing purposes.
In fact, concave cylinders should alone be used in making an examination, even where a complete trial-case is employed. To repeat one of the first rules of refraction: “As much plus or as little minus spherical power as patients will accept, combined with weakest minus cylinder, simplifies the work of refraction and insures accuracy without time-waste.”
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°.