[Illustration: Fig. 5—With supplementary disk pointer set at +6 Sph., this places an additional +6.D spherical lens at sight opening, extending instrument’s total convex spherical power to +12.D.] Should still greater power be desired, the small pointer at the outer edge of the instrument should be set at +6 sphere (Fig. 5). This controls a supplementary disk (Fig. 3c) which places an additional +6D. lens before the original range of lenses previously referred to, thus increasing the maximum power to +12D. If still greater strength is required, any additional trial-case lens may be added, a cell being provided for that purpose on the forward plate of the instrument. OPERATES AND INDICATES AUTOMATICALLY As previously explained, in using the Ski-optometer, it is only necessary to remember that each outward turn of the single reel toward the temporal side of the patient _increases_ the plus power, while the reverse turn toward the patient’s nose _decreases_ it. In fact, no attention need ever be given the register until the required sum-total is secured, it only being necessary to turn the single reel in order to be assured of the unvarying and accurate operation of the instrument. For convenience, the contour or upper edge of the plate covering the spherical reel has been made to fit the index finger (Fig. 3). Hence the operator should note that it requires but one complete turn from extreme side to side, rather than a number of short turns, in order to bring each individual lens into position, thus obtaining the full advantage of the automatic spring-stop. This likewise permits the refractionist to operate the Ski-optometer even though the room is in total darkness. CONCAVE SPHERICAL LENSES Another simple and exclusive Ski-optometer advantage worthy of note is the method employed in obtaining concave, spherical lenses. Instead of employing a battery of concave lenses similar to the convex battery previously described, the instrument’s operation is greatly simplified through the use of a neutralizing process. In short, the Ski-optometer only contains two concave lenses to obtain its entire series—namely, a -6.D and a -12.D sphere (Fig. 3c)—first setting the pointer of the supplementary disk at -6. sphere, then setting the indicator of the spherical battery at +6. Thus zero (or plano) is obtained, the plus neutralizing the minus. By merely turning the plus or convex spherical reel inward, or toward the patient’s nose, the convex power is then decreased, naturally increasing the concave value or total minus lens power. For example, if the spherical indicator shows +5.D, when the -6D. lens is placed behind it, the lens value at the sight opening will be -1D (Fig. 6). If required, the refractionist may continue on this plan until only the -6D. lens remains.