When Yoga is being performed, the forms which come first, producing apparitions in Brahman, are those of misty smoke, sun, fire, wind, fire-flies, lightnings, and a crystal moon.

In plain words: When someone practices Yoga, the first signs that appear in the mind are visions like mist, smoke, sun, fire, wind, fireflies, lightning and a clear moon. These show that the practice is moving toward Brahman.