We are skipping the section on hand neutralization in the text book. It is historically interesting, and it is an example of using the actual power/axis combinations to come up with a prescription. Read it through, and do the exercises if you find it interesting. You will not be tested on it in this class.

From now on, any dioptric power that does not have a + sign or a - sign is wrong. Whether it is right or not.

If the end result of a calculation is a power that you may order from a lab, retrieve from a stock drawer, or place in a pair of glasses, then the powers should be rounded to the closest eighth. +3.10 rounds to +3.12. -1.90 rounds to -1.87. What does +2.56 round to? Well, it is 0.06 from +2.50, and it is 0.06 from +2.62, so it is not closer to either one. In the lab class you will learn to always round up. Many Refractionists always round weaker. So, in this class, if the answer is exactly between two eighths, either answer will be acceptable.

0.00 is never used for the sphere, and if the cylinder is zero then you have a sphere so the cylinder is left off completely.

Use three digits for the axis, and do not use a degree sign. Why? because if you are writing 10 degrees (10) someone may mistake it for 100.

These are the prescription writing rules for Opticians who are taking this course. The last rule, of course, that goes without saying, (?) is to write neatly, so that other people can read the prescription without guessing. Guessing in this business is a no-no. Do not guess. Ask. Call. So the Rx did not have a sign for the sphere? The old glasses are -1.00 DS? So, call anyway, and verify that the sign is supposed to be a -. Do not assume.

Well, back to the point I started to make in that last paragraph. These are the rules in this course. They are not the Dr.'s rules. Dr.'s can write any way they wish. So can bosses. These rules are only for you and me.

Do the exercises on page (52 / 65-66), and call your instructor if you have any question on this.

On to the demonstration. Do you have that stock toric lens that I told you to get for this week?

First, look at the edges of the lens. The edges will not be uniformly thick or thin. When we were looking at spherical plus stock lenses the edge was the same 'thinness' all the way around; the minus lens was the same thickness. On this lens you will find that there are two sides where the edge is thickest and two sides where the edge is thinnest. The thickest edges will be opposite each other, and the thinnest edges will be 90 degrees away from the thick edges.

When we looked at plus and minus spheres for image movement we moved them right and left and watched the image of the edge of the paper. This lens will also have plus or minus power in its major meridians, so you will still get that kind of motion. This time hold it centered over the edge and rotate it clockwise and counter-clockwise.

See the edge rotate back and forth? We say that cylindrical lenses have scissors motion.

Notice that when the paper edge is lined up with the major meridians the line is straight. When the paper edge is in-between the major meridians the image is not on line. This is one way of finding the major meridians of a lens.