Trataka (Sanskrit n. त्राटक Trāṭaka: "to look, or to gaze") is a yogic purification (a shatkarma) and a tantric method of meditation that involves staring at a single point such as a small object, black dot or candle flame. It is said to bring energy to the "third eye" (ājňā chakra) and promote various psychic abilities.
By fixing the gaze the restless mind too comes to a halt. It is said also that control of the ciliary (blink) reflex stimulates the pineal gland, which Kundalini Yoga identify with the third eye. Trāṭaka is said to enhance the ability to concentrate. It increases the power of memory and brings the mind to a state of awareness, attention and focus.
The practitioner may fix attention on a symbol or yantra, such as the Om symbol, a black dot, the image of some deity or guru, a flame, a mirror or any point, and stare at it. A candle should be three to four feet (1 metre plus) away, the flame level with the eyes. Relax but keep the spine erect and remain wakeful and vigilant. The eyes begin to water. Some authorities recommend that the eyes should then be closed and the yogi concentrate on the after image, while others persevere with staring for 30–40 minutes. As per yoga and Tantric texts ,the eyes of one who got siddhi in trataka will appear like burning charcoal in colour. 
The Bihar School of Yoga, in India has published several books on meditation that give detailed instructions for practising trataka. Dharana Darshan by Swami Niranjanananda Saraswati has an entire chapter devoted to the practice.