The first pair of this kids of prescription eyeglasses was designed by Franklin, who added half reading segments into another half of primary Rx lens, and today, we have many different shapes of the reading segments, such as D shape and round one and so on, but they are usually account for a small part of the whole lenses, so, there is a crossing line between the two incompleteness lenses, that’s the reason why it cause frequent image jump, when this situation occurs from time to time, there comes the headache and dizziness, so, people may need a period to get used to their new bifocal reading glasses!