Gazing at a minute point with wink-less eyes till the tears appear is called Trataka. Its practice make one efficient in Shambhavi Mudra which shall be explained in the chapter of mudras. It is an excellent exercise of the eyes and improves their sight. The process is as follows :-
Make a black circle of one inch diameter on a wall at the height of 2.5 feet above the floor. Sit in front of it at the distance of about two feet and begin gazing at it with wink-less eyes and concentrated mind. After a few minutes, tears would appear. Shut the eyes for a brief rest. The process may be repeated two to three times. The daily practice of the Trataka would make it possible to look at the black spot for a pretty long time and with this the mental concentration and visual power would improve.