Abstract: A method for prescribing a prism factor for eyeglasses for substantially correcting vertical and/or horizontal diplopia or double vision problems, which includes having a patient positioned for gazing at a target image while holding their head in a substantially fixed position, and then closing their affected eye for a period of time, whereafter they are instructed to open the affected eye and immediately report upon where a double image occurs upon a measurement grid associated with the target image continuing to be observed by their other eye, for permitting a prism factor to be prescribed relative to the measured distance between the target and double image created by the defective eye.