 When you use a vertometer or lensometer, the first steps are to set the power wheel to zero, the axis wheel at 180 degrees, and the prism axis indicator at 180 degrees. Adjust the eyepiece until the target lines are sharp. Before mounting the lens, make sure the lens table is down and out of the way. Mount the lens so the convex side is facing you. Hold it so it looks to be centered against the lens tube. Now you're ready to take a reading of the lens power. Always keep both eyes open when using the instrument, otherwise you may get severe eye strain. Neutralize the lens by turning the power wheel until the target lines are sharply focused. Is this a plus reading or a minus? Plus. Remember that the black numbers mean plus. In school, you write down your power wheel reading. In an optical lab, you would check the reading against the prescription. Note that the power wheel is marked in one-eighths of a diopter in the lower range for plus and minus. Above three diopters, it's marked in one-fourths for plus and minus. Got that? Okay. What's this reading? If you said plus one and a quarter diopters, you're right. Looking through the eyepiece, you're able to accurately center the lens. Shift it until the middle line of each set of target lines crosses through the center of the reticle. The next step is to mark the optical center of the lens. Press down to ink the lens-marking device. Then press against the lens. Remove the lens. For this spherical lens, you only need the center dot, so wipe off the other two. Finally, mark it with an L or R as appropriate in an upper corner of the concave side. That completes the procedure for neutralizing a spherical lens. Let's run through it again with another lens. Mount the lens with the convex side facing you. And remember not to let the clamp spring against the lens. Turn the power wheel until the target lines are in sharp focus. Okay? What is this reading? The correct answer is minus two and a half diopters. Center the lens so the lines cross through the middle of the reticle. Mark the optical center of the lens. Wipe off the two outside dots and mark the lens. This completes the procedure for neutralizing a spherical lens.