The centre of all your psychic powers, of your conscious powers, the centres of these are in the astral, and if (especially with your senses, each of which has its own centre in the astral body) you overstrain the physical senses down here, you will get an action on the astral plane, but an unhealthy, because disorderly one, one not going along the line of evolution but trying to create from below instead of from above.

None the less, you may have some results, and in the two famous Indian systems for developing the powers of the consciousness, and for unfolding the consciousness itself, you have this recognised, and you read of Raja Yoga and of Hatha Yoga, of the Kingly Yoga and of the Yoga of Effort.

The Yoga of Effort is Hatha Yoga, and is practised by physical means and followed by physical effects.

The eye is stimulated in certain ways, and the effect of straining the physical eye is to bring about a certain limited kind of clairvoyance.

You can gain it in that way by gazing into crystals, and so on.