Another invisibility story! The number and variety of stories continues to amaze me.

You know what we haven’t really seen yet? A good invisible spy story! “Raiders Invisible” came close, but its invisible spies were the bad guys; we need a story about a heroic invisible spy!

So let’s take a look at “The Radiant Shell,” by Paul Ernst, which appeared in the January 1932 issue of Astounding Stories.

Paul Ernst is another “invisibility addict,” having written more than one tale about invisibility! His other story is “Beyond Power of Man,” which appeared in Weird Tales in 1928.

Quite frankly, “The Radiant Shell” is a very silly story. Let me quote the beginning paragraph to give you a sense of it:

“And that, gentlemen,” said the Secretary of War, “is the situation. Arvania has stolen the Ziegler plans and formulae. With their acquisition it becomes the most powerful nation on earth. The Ziegler plans are at present in the Arvanian Embassy, but they will be smuggled out of the country soon. Within a month of their landing in Arvania, war will be declared against us. That means”– he glanced at the tense faces around the conference table– “that we have about three months to live as a nation– unless we can get those plans!”

Oh, no! Not the Ziegler plans!

… uh, what are the Ziegler plans?

“The Ziegler heat ray is the deadliest weapon yet invented. A thousand men with a dozen of the ray-projectors can reduce us to smoking ruins while remaining far outside the range of our guns.”

So, the only hope the United States has to survive this deadly attack from *checks notes* Arvania is to steal the plans from the Arvanian embassy. And to do this, they agree to the assistance of Thorn Winter, a scientist who claims to be able to make himself invisible. I had to laugh at how readily the military agrees to this plan without any proof or demonstration of this invisibility:

“But… invisible….” muttered Forsyte, glancing askance at Winter.

“There’s no time for argument,” said the Secretary crisply. “The question is, shall we give this man permission to try the apparently impossible?”

All heads nodded, though in all eyes was doubt.

Thorn goes for a traditional H.G. Wells style invisible man looks, stripping naked to coat himself with his invisibility paint. How does it work? Thorn explains:

“Refraction of light,” said Thorn hurriedly. “The light rays strike this film, hurtle around the object it coats — at increased speed, probably, but there are no instruments accurate enough to check that — and emerge on the other side. Thus, you can look at a body so filmed, and not see it: your gaze travels around it and rests on objects in a straight line behind it.”

This is very much the classic sort of “invisibility cloak” scheme: light gets guided around the hidden object and sent on its way as if it encountered nothing at all.

Curiously, Ernst hits upon one challenge with the physics of a “perfect” invisibility cloak: light traveling through the cloak has to travel faster than the vacuum speed of light, because it has to make a detour to avoid the cloaked region. I use the following image to highlight this problem: light has to take the same amount of time to travel the two red lines, otherwise the cloak would in principle be detectable by the distortion of the image caused by the time delay. But the detour around the cloaked region means the light must travel a longer distance in the cloak, which means that it must travel faster than light outside the cloak.

But since we are usually trying to make things invisible in air, and the speed of light in air is very close to the speed of light in vacuum, light must travel faster than the vacuum speed of light in the cloak to make it work perfectly! It turns out that this is possible for a very narrow range of wavelengths, but that range of wavelengths of light is much, much smaller than the visible spectrum. You could make an object invisible, perhaps, for a certain specific shade of red, but not for all visible colors of light.

In the story, Thorn proceeds to the embassy and sneaks in to steal the plans, and twists and turns and narrow escapes happen. Ernst does introduce one challenge that many other authors of invisibility fiction never mention:

A little of the light cloud of dust stirred up by the truck wheels had settled over him and clung to the encasing shell. As he moved, these dust specks moved. The effect to the staring guard, Thorn realized, must be that of seeing a queer, fine dust column moving eccentrically over a grassy lawn where no dust column had any business to be.

In short: even if you make yourself invisible, you will get dirty, and that dirt will not be invisible!

Ernst also makes the… Arvanians… a reasonably worthy adversary. When they hear a sneeze and realize that there is an invisible intruder in their midst, they work systematically to capture and/or kill him:

The Arvanians split up into orderly formation. Two went to guard the door to the butler’s pantry, and two to cover the closed sliding doors to the outer hall. Six, with drawn swords sweeping back and forth before them, walked slowly toward the wall from which the sneeze had come.

Overall, “The Radiant Shell” is just a very silly story. You can read it in pdf form here if you like. It is, at least, like the “Invisible Robinhood,” one of a very rare class of stories where the invisible being is actually the hero!