Used in vision tests, the Maddox rod lens has a number of molded glass grooves (cylinders) through which a spot of light appears as a streak perpendicular to the cylinders axis. Our understanding is that they are used to uncover latent squints (phorias). When a distant spot of light is viewed with the Maddox lens in front of one eye, a line will be seen by the eye covered with lens, while a spot of light is seen with the other eye. The line will be at right angles to the grooves. When the grooves are horizontal, the line will appear vertical. Someone with normal vision will see the line pass through the spot of light, but someone with latent squint will see the kight source to one side of the line. 13mm diameter by 3mm thick. Unused.