It is not widely known that Hitler was a vegetarian, and even fewer people know that he was also an animal welfare advocate. Allegedly, the idea to ban bowl-shaped aquariums originated with him, as he believed it was animal cruelty that the fish had no corners to hide in and that the bowl distorted the view, damaging the fish’s intellectual development. His idea outlived him; in fact, there is a country where it was enshrined in government regulations as late as 2010. Was the Führer right, or is it all nonsense?
It is well known that Hitler was a vegetarian.[i] Not for health reasons, but because of his ecological paranoia. As Thomas Malthus pointed out, Earth’s land is finite, and as population grows, resources become scarce. According to the Führer, the struggle for land and food determines whether a species prevails or perishes. For him, struggle was not a means to an end—it was life itself, “our daily bread,” as he wrote in Mein Kampf.[ii] He could not hold his own people in high regard either; he considered only the Germanic race fit for the fight. To him, the triumph of Jewish intellect over brute force signaled the end of struggle and the extinction of species.
The first animal protection law
Hitler’s ideas on animal protection were not born out of affection for furry creatures but rather from viewing them as instruments in the eternal struggle.[iii] One of his bizarre ideas was the “Wooffen SS,” an elite dog unit in which super-intelligent dogs would perform tasks alongside—or instead of—SS officers. Across Germany, Nazi officials collected dogs deemed suitable, and at the Tier-Sprechschule (Animal Speech School) near Leutenburg, they attempted to teach them to talk. Yes, you read that correctly: to talk. Allegedly, the most successful dog was the one that could say “Mein Führer.” Annoyingly, it was a mixed breed, not purebred.
After this, one is hardly surprised to learn that Hitler tested the effects of a cyanide capsule on one of his favorite German shepherds, Blondi. Sadly, this is true. What is not true, however, is that the first animal protection law was thanks to the Nazis. The biblical Sabbath law prescribed rest for animals as well, and the question of animal welfare was already on the agenda in ancient Greece. The first modern animal protection law is credited to Richard Martin, an Irish politician nicknamed “Humanity Dick.” After it came into force in 1822, similar laws began to appear in America, England, and France. The first animal protection society was founded in 1824 by Reverend Arthur Broome, one of Martin’s enthusiastic followers; this was the still-active RSPCA (Royal Society for the Prevention of Cruelty to Animals). So, it is also untrue that Hitler founded the first animal welfare association. In fact, the Nazi regime dissolved all existing nature, environmental, and animal protection organizations[iv], as such forms of civic organization were seen as potential enemies of National Socialism. However, it is true that shortly after the Nazis came to power, in April 1933, they enacted an animal welfare law (Tierschutzgesetz), which bore many similarities to the British law.[v] The law currently in force in Germany is still quite similar, only without Nazi peculiarities like the ban on kosher slaughter.[vi]
One of Hitler’s excesses was the ban on fishbowls. In Season 2, Episode 4 of Mythbusters by Adam Savage and Jamie Hyneman, it was mentioned that Hitler banned fishbowls because he considered them cruel: they lacked corners for fish to hide in, and the spherical shape distorted the view, allegedly harming the fish’s intellectual development. It’s easy to see that if the bowl is large enough (for example, 4–6 liters is sufficient for a betta), and if it is well-decorated with plants, wood, and rocks, there will be plenty of hiding spots. But what about the distortion? When we look through a fishbowl, the world behind it is severely distorted—but what does the fish see from inside the water of the bowl?
A Light Beam Bathing in Milk
From our school studies, we know that when a light beam strikes the surface of water, part of it is reflected, while another part penetrates into the water and is refracted. This can be demonstrated quite well by filling a glass container with water and adding a few drops of milk. The incoming beam forms an angle α with the normal to the surface; the reflected beam forms an angle α′, and the refracted beam forms an angle β (in optics, these angles are measured relative to the normal). According to Snell’s law (also known as the Snellius–Descartes law), regardless of the angle at which the light strikes, the ratio of the sines of α and β remains constant:
sin(α) / sin (β) = n12
…where n₁₂ is a constant material property, the relative refractive index of the two media. For example, the refractive index of freshwater relative to air is about 1.33. When light enters an optically denser medium (in this case, water), the beam bends toward the normal, meaning that α > β in every case. But what is β when α is close to 90°, that is, when the light arrives almost grazing the water surface? Since sin(90°) = 1, we have: sin(β)=11.33 which gives β ≈ 49°. In other words, light that enters the water almost parallel to the surface continues at about a 49° angle relative to the normal.
What does this mean? It means that any light coming from above reaches the fish’s eye within a cone of 49° half-angle. Outside this cone, the outside world is invisible. Inside it, however, the entire hemisphere above the water surface is compressed into this circular window.
Thus, the fish sees the whole world above the water as a panoramic image through a round “window.” This phenomenon is known as the Snell’s window, something divers are very familiar with.
Note: Willebrord Snellius (born Willebrord Snel van Royen, 1580–1626, known in the English-speaking world as Snell) was a Dutch astronomer and mathematician. For a long time in the West, the law of refraction was attributed to him (the Snell’s law, or in textbooks influenced by French and Prussian traditions, the Snellius–Descartes law). Today, however, we know that the law was first written down by a lesser-known Muslim mathematician, Ibn Sahl of Baghdad, around 984 CE. His partially preserved work was an important precursor to Ibn al-Haytham’s (Alhazen’s) Kitab al-Manazir (Book of Optics), written about thirty years later. This influential treatise appeared in Europe over five centuries later in a Latin translation under the title Opticae Thesaurus Alhazeni Arabis.
According to Wikipedia, Alhazen did not know the law of refraction, but this is incorrect. He certainly knew Ibn Sahl’s work and thus the law, although his own book paid greater attention to the nature of human color and shape perception, where he made original contributions. When discussing reflection from concave and convex mirrors and the refraction and magnification of lenses, Alhazen began to outline a more general principle, one that could also account for light bending not only at the boundary between two media with different refractive indices but also in inhomogeneous media with gradually changing refractive indices (e.g., mirages, sunsets).
This principle, now known as Fermat’s principle of least optical path, states that a light ray always follows a path such that the travel time between two points is an extremum (in practice, usually a minimum). Alhazen did not explicitly state the principle in this form, but when solving the so-called Alhazen problem—finding the reflection point on a spherical mirror—he essentially applied it by using an iterative method to solve a quartic equation. His solution was so complex that later translators failed to understand it, and it was omitted from the Latin version. For a clear explanation of the Alhazen problem, see: http://tananyag.geomatech.hu/m/zdEAeP9J (in Hungarian).
Thus, Snellius, Descartes, and Fermat—although they undoubtedly relied on results achieved by Muslim scholars half a millennium earlier—had to “reinvent” these laws. If we insist on priority, we might today call Snell’s window the Ibn Sahl window, but in recognition of the Dutch scientist’s contributions, the term Snellius window remains appropriate.[vii]
Model Experiment…
A typical rectangular aquarium, say 120×45×45 cm, usually has three vertical sides exposed to the outside world. This means that in addition to the natural upper Snell’s window, there are also three lateral Snell’s windows, in which the surroundings (the room) appear in panoramic form.
The Ray Optics Simulation tool is an easy-to-use web application designed for modeling the behavior of mirrors and lenses, but with a little ingenuity, it works perfectly for our purpose. Launch the app from https://ricktu288.github.io/ray-optics!
- In the top right, turn on the Grid and Snap to Grid options.
- From the Glass menu, select Polygon / Circular Arcs, then draw an elongated rectangle in the center of the screen.
- On the top, set the Glass Refractive index value to 1.33.
The program now assumes we are dealing with a solid glass block with the same refractive index as water. We won’t put a fish inside—but let’s just pretend this is our aquarium.
One more principle we need for the modeling: in geometrical optics, light paths are reversible. This means that light rays entering the fish’s eye from outside can just as well be represented by rays emanating from a point source inside the aquarium, because they follow the same path in reverse.
So, instead of a fish, choose Point source from the Light Source menu and place a point light source inside the aquarium close to one of the walls! On the top set the Ray density slider to make the program display plenty of rays.
Now the positions of the lateral Snell’s windows become clearly visible: the rays crossing these “windows” cover 180° of the external world. The program also calculates rays that reflect off the inner walls. For us, this simply means that, the fish does not see darkness around the Snell’s window but the reflections of the aquarium’s other sides.
Let’s examine the end panels more closely! Move the point source toward one corner. You’ll see that the fields of view of the side and end Snell’s windows overlap. Anything near the aquarium corner appears doubled to the fish. And the reverse is also true: if we stand at an outside corner of the aquarium, fish swimming near that corner appear duplicated.
In other words, the corner is not a hiding spot—visually, it’s an especially unpleasant place.
With the method we’ve just learned, modeling a spherical fishbowl is child’s play—just draw a circle instead of a rectangle. The result is surprising! We see no ray patterns resembling a Snell’s window at all. What’s more, the rays passing through the aquarium wall are bent much less than in the case of a flat-walled aquarium.
What does this mean? It means that a fish sitting in a spherical aquarium can see almost everything in the surroundings that isn’t blocked by objects inside, and it does so with less distortion than in a rectangular aquarium.
(Note: If you move the point source too close to the sphere’s wall, a bright ring appears with the message “Processing… Click here to stop.” This happens because, under certain conditions, light undergoes total internal reflection in a narrow zone, going around endlessly. Of course, in reality, it doesn’t go on forever—the light eventually gets absorbed—but the program can’t account for that, so it would calculate the reflections indefinitely if you allowed. Interestingly, this is the very principle exploited in fiber optic cables, where the goal is to keep light trapped inside the glass fiber.)
…and the real-world experiment
If you have a spherical aquarium and a waterproof smartphone, you can easily verify what the fish actually sees. Another option is to anchor a reflective sphere (for example, a Christmas ornament) at the bottom of the aquarium and photograph the panorama reflected on the sphere from above, using a telephoto lens aligned approximately perpendicular to the water surface.
I chose this second method. The observations perfectly match the simulation results. In a rectangular aquarium, the mirrored sphere placed next to a side wall clearly shows the lateral and upper Snell’s windows. The vertical edges of the aquarium are also visible, making it easy to identify that outside the Snell’s window, the reflections of other aquarium walls appear. In this visual chaos, multiple reflections can also be spotted.
When the mirrored sphere is placed in the spherical aquarium, the reflection shows a relatively normal circular panorama. The water surface is, of course, still flat, so there remains a Snell’s window at the top, in which objects near the edge appear doubled. In practice, however, serious spherical aquariums come with a lid, lighting, and water filter, which completely covers this upper window, making it disappear.
Looking sideways, we see that the outside world appears much less distorted; objects are roughly in the direction where they actually are, and there are no multiple or mirror images at all. The external view is clear and coherent, not chaotic as in a rectangular aquarium. Thus, in terms of spatial orientation, a spherical aquarium is more favorable than a rectangular one.
From my personal experience, however, neither aquarium shape seems to bother the fish—they recognize quite well when someone approaches who usually feeds them. What absolutely should not be used for keeping fish are brandy-glass-sized bowls with unfiltered water. These must be banned—but for their size, not their shape. Whether square or round, a 2-liter aquarium is not a suitable living space for any mature fish.
- In a 12–13 liter tank, you can try keeping snails or shrimp.
- In a 25–30 liter one, you can keep gudgeons or labyrinth fish.
- For most other fish, a livable aquarium is at least 80–120 liters, and for goldfish, it should be 100–200 liters.
- Spherical aquariums are manufactured up to about 30 liters (rarely 50 liters), which is already suitable for fish. The smaller 1–12 liter bowls are best reserved for plants only.
Miért tévedett Hitler?
Hitler believed that the spherical shape distorted a fish’s vision and thus harmed its “intellectual development.” In reality, what the fish sees is not what we see when looking through an aquarium. The fish looks outward, and in a rectangular tank its view is defined by the Snell’s window phenomenon. In a spherical aquarium, this effect does not occur—the spherical shape does not degrade the outside view.
When a human looks through an aquarium, parallel glass panes make the far side of the world appear fairly normal, whereas a spherical aquarium behaves like a strongly distorting magnifying lens. But we must not forget that in this case, light passes through two water–glass–air boundaries, producing a completely different image than what a fish sees from inside.
Hitler assumed that visual distortion caused “intellectual damage” in fish. But a fish’s brain does not process images like a human’s. Over millions of years of evolution, fish have adapted to refraction and underwater optics. Unlike us, they have no concept of the geometry of the outside world, so it doesn’t bother them that things look different from reality. When you approach the aquarium, they become lively, expecting food—but don’t fool yourself into thinking they recognize you. They just see movement. Try this: stick a mop near the tank and watch. The reaction will be the same as when you show up. To them, you are the mop—and they don’t care.
Much has been debated about whether Hitler was insane. Some say he wasn’t mad at all; on the contrary, his success lay in how much he resembled ordinary people. One thing is certain: he was foolish enough not to grasp the optics of a spherical aquarium. The city council of Rome banned spherical aquariums in 2005 by municipal decree, and the Hungarian government followed with Decree 41/2010 (II. 26.). The lesson? It’s never good when power falls into the hands of fools. Hitler was too stupid to truly understand Darwin’s theory—yet bold enough to exploit it, and in that case, it wasn’t just goldfish that paid the price…
References
[i] Adolf Hitler and vegetarianism – Wikipedia
[ii] Mein Kampf, p. 281; Zweites Buch, pp. 15, 74.
[iii] Hitler: Table Talk, pp. 51, 141.
[iv] Boria Sax (2000). Animals in the Third Reich: Pets, Scapegoats, and the Holocaust. Continuum International Publishing Group. p. 41. ISBN 0-8264-1289-0
[v] Animal rights – Wikipedia
[vi] Bundesgesetzblatt – German Animal Protection Law PDF
[vii] Zghal, Mourad; et al. (2007). “The first steps for learning optics: Ibn Sahl’s, Al-Haytham’s and Young’s works on refraction as typical examples.” Education and Training in Optics and Photonics Conference. International Commission for Optics: 3. Retrieved 20 June 2011. (PDF link)