Urza - Time Machine Operation

“You must stand here, within the circle,” said Master Malzra. “That is all. You will stand and wait while the machine does its work. When the time regression slows, you must step from the circle to arrive in the former time. You will remain somewhat out of phase in the former time—you will be able to see your surroundings, but no one will be able to see you. This is to protect the time continuity. As the particles of your being gradually align themselves with your surroundings, this out-of-phase effect will lessen, and you will become visible. Whether in phase or out, you will be able to affect your environment, but we ask that you make no significant alterations, again for sake of temporal integrity. We will control your return trip from this end. When you are drawn back to the present, you will make a report of what you have discovered. Do you understand?”

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Time Streams, Chapter 2