The polished mahogany chamber, with its gleaming brass fittings and precisely ground lens, sat proudly on the Dauphin's table like a jewel of science. For weeks now, Louis-Joseph had admired its form, even run his fingers over the delicate tube that held its convex glass. To the courtiers it was a plaything; to Louis XVI it was a father's indulgence; but to the boy himself, it was a cornerstone.
And yet, very quickly, he saw what he had expected all along. The image it produced—though wondrous to eyes accustomed to nothing but fleeting shadows—was far from what he required. Edges blurred into haze, faint halos of color bled around fine details. The device was impressive, but crude. It would not suffice.
The Dauphin, knowing well how to play his role, expressed his discontent in the only manner that would rouse his father's protective pride.
"Father," he said one morning, peering through the eyepiece with exaggerated frustration, "it's all blurry! And look—there are rainbows everywhere! My microscope is broken!"
Louis XVI, seated nearby with a half-finished lock upon his workbench, looked up at once. To most, the words of a child would have been dismissed as impatience. But this was his son—the heir to France, and more importantly, a boy who shared his own passion for mechanics and discovery.
"Let me see," the King said, taking the instrument into his large hands. He leaned close, his own eye pressed to the lens. A moment later he frowned. "Hmm. You are right. There are colors. And the image is sharp only at the center. This will not do."
The boy allowed himself a secret smile. His father had taken the bait.And this could be considered an anniversary present to his father.
Louis XVI did not waste time wondering what must be done. He was no philosopher lost in abstractions, but a craftsman and engineer at heart. His mind moved in practical lines. The question was not what to do, but whom to call.
Within days, the palace stirred with new arrivals. The lunetier from the Observatoire, already familiar with the palace, was summoned again to present his craft. Letters were dispatched to the Académie des Sciences, requesting the attendance of its learned men. Among them came Alexandre-Théophile Vandermonde, a mathematician with a passion for instruments, and Jean-Baptiste Le Roy, renowned for his electrical experiments but well-versed in optics.
When they assembled in the royal workshop, Louis XVI laid out the matter plainly, gesturing toward the chamber.
"Messieurs," he said, his deep voice carrying the authority of both king and craftsman, "this instrument is promising, but it fails in clarity. Observe: the edges are blurred, and there are garish fringes of color. The boy calls them rainbows, and he is right. They are intolerable."
The scholars nodded knowingly. Aberration chromatique—the splitting of light into colors—was a well-known problem, as was the spherical distortion that bent lines near the edge of the lens. John Dollond's achromatic lenses in England had offered solutions, but such designs were not easily replicated in France.
Louis XVI pressed them further. "The image is bright at the center, dull at the sides. The lighting is weak and uneven. Tell me, can we improve it? Can we make the field sharp and clear from edge to edge?"
As the learned men debated among themselves, Louis-Joseph sat silently, feigning the curiosity of a child. But within, his mind blazed with memories. He had seen diagrams in his former life, arguments traded between amateurs and professionals alike. He knew the very words they now spoke—chromatic aberration, spherical aberration, condenser—though he carefully masked his familiarity.
At last, when the moment seemed right, he leaned toward his father with innocent eagerness.
"Father," he said, "what if the light itself were the problem? If the image is dim, perhaps we could gather more of it? Look—" He sketched quickly on a scrap of paper, a crude drawing of a simple convex glass focusing rays toward a point. "A lens here, before the sample. Like a magnifying glass, but only to make the light stronger. Wouldn't that help?"
The savants exchanged glances, surprised. Vandermonde muttered something about "a condenser lens," while Le Roy stroked his chin. The idea was not unheard of, but it had not yet become standard practice. That the notion should spring from the lips of a child astonished them.
Louis XVI's eyes lit up with paternal pride. "You see, messieurs? Even my son sees further than some men! Yes, a lens to gather the light. Let us attempt it."
The Dauphin hid his triumph behind a modest smile. He had introduced, with a few strokes of charcoal, a principle that in his own century would not become universal until long after. And no one suspected.