Patanjali's Yoga Sutra // 1:36

concentration on a painless inner state of lucidness and luminosity also brings stability and tranquility

// The easiest way to practice this is to place your attention in the space between the breasts, the heart center. Simply imagine that there is a glowing luminosity there, about the size of the palm of your hand. Whether or not you literally see with your inner eye is not important; the practice works either way. Maintain an inner attitude that it does not matter what other thoughts, images, impressions or memories might arise in the mind field; you will hold that stance that these will not disturb or distract you. Stay only with that glowing inner luminosity in the heart.

Emma Wallace