”Through concentrating on the Lord a symptom of perfection may appear before the practitioner, and the Lord upon whom meditation is being performed may sometimes appear in the heart like that of a fog or mist or sun or wind or fire or fire-fly or as lightening or as a crystal stone or as a moon. These objects, if seen in the heart while in meditation, are indications of success in yoga.”
khadyota vidyut sphatika sasinam
etani rupani purahsarani
brahmany abhivyakti-karani yoge
PURPORT BY SRI KRSNA BALARAM SWAMIJI
Seeing the symptoms an intelligent or a learned person can recognize the indication of upcoming things. According to the Yogavasisthya, if a yogi meditates following the authentic scriptural rules then the symptoms of success can be visualized on their body. If the yogi saw the Lord in the heart in the form of fog the external physical symptom will seem to appear in darker complexion, if the Lord was seen as mist then the body would appear lean, if Lord was seen in the form of the sun the body would have vigor. If the Lord was seen as air the body would have some pain and if the Lord was seen as fire the body of the yogi will appear reddish. If the Lord was seen as a firefly the body will appear yellow. If the Lord was seen as lightening then the body of the yogi will experience some electrical energy. If the Lord was seen as a crystal stone then the yogi would appear very pleasing and if the Lord was seen as a moon the yogi will experience cold as if bathed in ice water. Such symptoms occur when the yogi has success in meditation. If such experiences are not felt then it should understood that the yogi is yet far from success.
[Svetasvatara Upanisad, 2.10, KBS0016]