Desperate, Yalda blurted out an idea that had been lurking unvoiced in the back of her mind. “Sir, forgive me, but… what if you fed an entire star trail into your device? Properly focused, wouldn’t the spread of colors be recombined into an image that was bright enough to see?”
Giorgio said, “Please! Our guest is tired!”
Nereo raised a hand, requesting his forbearance, and responded to Yalda. “By the principle of reversibility, yes—but only if the way the colors were distributed by the two methods were in precise agreement, which I doubt would be the case.”
Yalda’s skin tingled with excitement. If the recombination of the colors was sensitive to the detailed way the light was brought together, all the better.
“What if the star trail was focused by means of a flexible mirror?” she suggested. “A band that could be adjusted to vary the angles at which the colors were delivered, all along its length. By changing the shape of that mirror until the combined system yielded a single, sharp image for the star… wouldn’t the final, successful shape embody information about the relationship between wavelength and velocity?”
Nereo fell into a thoughtful silence. Rufino stared at the floor, embarrassed. Giorgio stared directly at Yalda; she could tell that he was actually quite taken by the sheer audacity of her suggestion, if not by the clumsy way she’d raised it.
Nereo said, “It just might work. And if you blocked the center of the star’s image—the brightest part—your eyes would adapt to the lesser brightness of the remaining halo, allowing you more easily to judge when your adjustments had diminished it as much as possible.”
Yalda was momentarily lost for words. If Nereo was offering her ways to improve her methodology, that meant he was taking the whole thing seriously.
“So you think it’s a worthwhile experiment?” she asked.
“Absolutely,” Nereo assured her. “And better you than me on Mount Peerless! I’ve grown used to the presence of certain decadent comforts, such as air.”
Giorgio buzzed amusement.
Yalda had never been to the observatory, but she didn’t care what hardships it entailed. “You’ll let us borrow the light comb, sir?” The glittering key to the secrets of wavelength, bought with his patron’s incomparable wealth, would be borne up the mountain’s slope to meet the starlight… in her hands?
“I’ll let you borrow it for eight chimes,” Nereo replied, “before I have to leave to catch my train. That should be long enough for you to calibrate your best prism against it.”
“Prism? But—”
Nereo said, “Everything in your methodology should work just as well with a prism used to recombine the star trails; all you need in order to make that worthwhile is a conversion table that translates between the angles at which the same hue emerges from the different devices. Do you think you can manage that, before I depart?”
In the optics workshop a young student was using the heliostat for an experiment in polarization, but when Yalda asked if he could take an early lunch he obliged without a moment’s hesitation.
From the storage room, she chose a prism that she’d used before; the sides had been polished to near-perfect flatness, and were unblemished by chips or scratches. Equally, the clearstone from which it had been cut appeared to contain no internal flaws. She knew that it would separate the colors smoothly, however mysterious its method.
With the prism in place in the beam of sunlight that was brought down from the clockwork-driven mirror on the roof, Yalda locked Nereo’s comb onto a platform that could swivel through the emerging fan of colors, along with a slit for selecting a narrow range of the prism’s light. The slit could not be set too fine, though, or it would itself diffract the beam.
She placed a white screen half a stride beyond the comb, and set about recording the pairs of angles for a succession of hues: the angle at which the light had been bent by the prism, and the angle at which it was subsequently bent by the comb.
Yalda worked with scrupulous care, but after a while the process became mechanical, automatic. She glanced at the polarisers she’d taken off the bench: slabs of an exotic form of clearstone from Shattered Hill. Place one of them in a beam of light, and the beam’s brightness was diminished by one third. A second polariser aligned identically with the first had no effect, but if the two were “crossed”—their axes arranged at right angles to each other—the original brightness was diminished by a further third.
Giorgio had sought to explain this in terms of the wave doctrine. An elastic solid could experience shear waves, in which the medium suffered distortions perpendicular to the direction of the wave’s motion. A polariser, he argued, must somehow be inhibiting light’s equivalent of such waves when they lined up with the stone’s special axis. A horizontal polariser could rid a beam of sunlight of its left-to-right vibrations; a second, aligned vertically, would rid it of all waves that vibrated up and down.
A mystery remained, though. Along with shear waves, every solid carried pressure waves, which were much like the sound waves in air. The velocities of the two kinds of wave were due to distinct properties of the material, and pressure waves always traveled faster than shear waves. It would require both a truly bizarre material and an absurd coincidence to force the two to share the same speed.
When two crossed polarisers were held up to a star trail, if the light that emerged had traveled from the star at a different speed than the light that was blocked, some portion of the trail’s spread of velocities should have been favored over the rest. But in fact what was seen was a perfectly uniform dimming of the entire trail. Light waves that lacked polarity—supposedly the equivalent of a solid’s pressure waves—were no faster or slower than the rest.
Yalda could not believe that this was a coincidence—a perfect conspiracy of elastic moduli. Rather, what it suggested was that the whole analogy was flawed. Whatever carried light between the stars wasn’t actually being squeezed and stretched and sheared. Nereo had pinned down light’s wavelength, the distance at which each cycle repeated, but the truth was that no one yet had an answer to the question: cycles of what?
When Yalda had a full range of measurements, she sprinkled dye onto her chest and made three copies of the figures on paper: one for Giorgio, one for Nereo—not much use to him, since the numbers were tied to a particular slab of clearstone, but an appropriate gesture nonetheless—and one to keep in the workshop alongside the prism.
Nereo was waiting at the university’s southern gate, an ornate stone archway encircled by violet-flowered vines that had been bred to open their blossoms even in daylight. Yalda thanked him profusely, and almost offered to carry his luggage to the station, but Rufino and Zosimo were already grappling with the cases, and she’d learned not to wound their pride with gratuitous displays of physical prowess.
When they’d left, Giorgio upbraided her sternly before finally conceding, “I suppose it was worth it in the end. You’re not much of a diplomat, but this could yield interesting results.”
The understatement was insulting, but Yalda didn’t push her luck. “I hope so,” she said.
Giorgio regarded her with weary affection. “And I hope you’re ready to try displaying a bit more tact.”
“Of course!” Yalda said, genuinely chastened now. “The next visitor, I promise—”
Giorgio hummed, annoyed. “Forget about the next visitor! You want to use the telescope, don’t you?”
“Yes.” Yalda was bewildered; did he mean that she’d probably be up on the mountain, unable to cause any more embarrassment when his next guest came to speak?