 You already know how to neutralize spherical lenses. You've seen how the target lines come into focus as you adjust the power wheel. But with cylindrical lenses, there's an important difference. Because the lens has two different powers ground into it, the two sets of target lines come into focus at different power wheel settings. So, unless you follow the procedures exactly, you may get entirely wrong readings. For example, let's take a reading when the thin lines are in focus with the axis wheel set at 180 degrees. It's minus three diopters. But now suppose you do the same thing when the axis dial is set to 90 degrees. Here's how this movement looks through the eyepiece. Neutralize again for the thin lines. This time, you get an entirely different answer, minus two diopters. You must learn the correct step by step procedure for neutralizing cylindrical lenses. First, we'll show the procedure for neutralizing a single vision glass lens, working in plus cylinder form. Turn the power wheel to the full minus setting. It's not hard to remember. Think opposites. Plus cylinder, use a minus setting. Now, turn the power wheel until one set of lines comes into focus. This should be the thin lines. If it's not the thin lines that come into focus first, turn the lens 90 degrees until you get a sharp focus on the thin lines. Center the lens. Read the power wheel setting. This is the spherical power of the lens. Write it down in plus cylinder form. Next, rotate the power wheel farther in the same direction until the broad lines come into focus. Center the lens. You've gone from minus three to minus two or one diopter in the plus direction. Therefore, plus one is your cylinder power. Another way to figure this is the method you've been using in class. Remember, you are not adding and subtracting numbers as usual. You are just finding the distance between the first and second readings. Note the axis dial setting. 180 degrees. Write it down as it would appear on the prescription. You've now finished neutralizing the lens. All that remains is to spot and mark it. You do not wipe off the outer dots since they mark the 180th meridian. The next lens we'll do is a bifocal. Mount it with the bifocal segment at the bottom and below the lens tube. So you are looking through the lens itself, not the segment. Convex side is toward you as usual. With bifocals, you always work in minus cylinder form. So you start with the power wheel, turn to the full plus setting. It's not hard to remember. Think opposites. Plus cylinder use a minus setting. Minus cylinder, a plus setting. As before, you turn the power wheel until one set of lines comes into focus. Again, it's the thin set of lines you want first. Suppose you get the thick lines instead. With a bifocal, you do not turn the lens since you want to keep the segment at the bottom. What you do is turn the axis wheel until the thin lines come into focus and are unbroken. Note the reading. Then refocus the power wheel as necessary. Center the lens by hand until the target lines cross in the center. Do not rotate the lens. You're now ready to record the sphere power value. Plus two diopters, which is your first reading on the power wheel. And the axis value, 90 degrees. You will not reset the axis wheel for cylinder power. Leave it right where it is. Now you rotate the power wheel in the same direction as before until the thick lines come into focus, noting how far you move it. In this case, you've gone from plus two to plus one or one diopter in the minus direction. You write this down as the cylinder power in minus cylinder form. After checking to see if the lens is still centered, you spot the lens. Now you're ready to determine the add power. Turn the power wheel back to your first reading, the sphere power value, which is plus two. Slide the lens up so the bifocal segment, also called the add portion, is against the lens tube. Here's how this movement looks through the eyepiece. Now turn the power wheel toward the plus numbers until the thin lines come into focus. How far you have to turn the power wheel from the sphere power setting is the value of add power. The power wheel has gone from plus two diopters to plus three and one half diopters, which gives you a value of plus one and one half diopters. This completes the neutralizing of a bifocal lens.