Looking past her toward that far pillar of frosty light where all the worlds of men were lost, he added hopefully,
"It's that psychophysical factor that makes it the equation of clairvoyance."
"You mean you can see with it?" she whispered. "Like Mr. Overstreet?"
"I hope so." He nodded thoughtfully.
"If I can learn how to use it.
Because the space factor vanishes when you solve for the psychophysical term, and the factor of past time is infinitesimal.
The only actual limit is a factor of uncertainty, which increases to infinity in future time."
She shook her head reproachfully.
"That means," he tried to explain, "the equation tells how we ought to be able to see anything happening anywhere, right now - except of course in a place shielded with a powerful psychophysical field, the way that new grid is.
We should be able to see things that happened a long time ago, though that would be harder.
But things that haven't happened yet will be dim and uncertain, because of that factor of increasing improbability, and I don't think we can ever hope to see far ahead."
"That doesn't really matter."
Her puzzled eyes had brightened slightly.
"If you can just find Mr. Ironsmith - and then go on to help poor Mr. White."
Sitting at that little desk beneath the dome Forester lifted his brown, wistful face again to that cloud of stars beyond the dead valley.
His searching eyes saw only misty light already old before men first thought of crossing space, but his mind explored that binding medium where distance was no barrier and even the veil of time drew thin.
The anxious child saw him nod at last, and the empty vagueness of his straining eyes turn to sharp attention.
"Do you see him?" she whispered huskily. "Mr. Ironsmith?"
"It's hard to see anything." Still he faced that toppling column of cold cloud, his voice slow with effort.
"The equation tells the method, but I haven't learned the skill.
It's hard to focus the perception - with all the universe in view.
Hard not to see too much."
But he looked again, and presently she saw his faint smile of triumph.
"Yes, I've found Frank Ironsmith now." His voice was so low she had to lean across the desk to hear.
"Back in the past.
Back at Starmont, before the machines ever came.
We must follow him down through time, and trace him when he leaves-"
Forester shuddered, and something hardened that pale smile into a grimace of pain and hate.
His bald head sank forward and his drawn face turned gray and his thin lips whitened.
The child recoiled a little from him, before she asked gently:
"What did you see that hurt you so?"
"Ironsmith and - Ruth." His terrible eyes focused on her face for an instant, and then looked searchingly back toward the leaning, luminous plume.
"That doesn't matter now - except to him and Ruth and me."
His voice was harsh and slow. "We must follow them from Starmont - though it's hard to trace the world lines, where they are teleported."
Jane waited, watching the changes on his haggard face.
She saw effort and agony and dread, but at last he nodded again.
"I've found it." Still he looked at the leaning galaxy, and still his voice was hoarse with strain. "The den of the human renegades." He shook his head uneasily.
"Though I still don't understand that Compact."
Shivering, the child stood watching. After a long time, the man came back to her in the silent cupola.
Drawing a long tired breath, he smiled a greeting to her and then stood up to stretch, flinching when he put too much weight on his knee.
"Did you find Mr. Ironsmith?" she whispered. "Now?"
"I followed him and Ruth, and I found the traitors' nest." He began limping restlessly about the gloomy cupola.
"I saw them there together, a few days ago.
But now he's gone - I don't know where." His baffled eyes flickered back toward the far galaxy.
"Still on Wing IV, I imagine - probably helping the humanoids complete that platinum brain.
But I'm afraid to look for him there any more, because I felt the potential of it when I tried."
His thin frame shivered. "The psychophysical energy of that mechanical brain," he breathed harshly, "reaching out to operate every man alive - and already terribly strong."
"Then what - what can we do?"
"I looked ahead." His voice was shaken with the conflict of fear and resolution.
"That factor of uncertainty makes things blurred and dim, but I think I saw him coming back to Ruth.