Study a single sutra by selecting the pada (chapter) and then the sutra (verse).
Source: English translation from The Yoga Sutras of Patanjali (Sri Swami Satchidananda)
By samyama on the light at the crown of the head (sahasrara chakra), visions of masters and adepts are obtained.
Source: English translation from Light on the Yoga Sutras of Patanjali (B.K.S. Iyengar)
By performing samyama on the light of the crown of the head (ajna cakra), the yogi has visions of perfected beings.
Source: How to Know God: The Yoga Aphorisms of Patanjali (Swami Prabhavananda, Christopher Isherwood)
By making samyama on the radiance within the back of the head, one becomes able to see the celestial beings.