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In Greek mythology, Hades' magic cap or helmet renders its wearer invisible. The most famous user of this piece of equipment is Hades' nephew, the hero Perseus, while on his mission to slay the Gorgon Medusa.

For the mission, Perseus was outfitted with some other magical apparel, including a sword (or sickle, or sickle-shaped sword), a pair of winged sandals, and a bag in which to carry Medusa's severed head. Together with these he is sometimes said to have had also a polished shield to use as a mirror in order for him to be able to look at Medusa's reflection, which was harmless, and thus see her without the danger of direct eye contact.

I imagine that an invisibility cap is quite useless if it doesn't also hide the clothes and shoes that one is wearing in addition to the rest of him. So presumably Perseus' clothes (and bag and winged sandals?) went invisible when he wore Hades' cap. But what about his sword/sickle, or something as big as I imagine his shield was?

And if the shield did become invisible because he was carrying it, how could he use it? Could he himself see it (and the reflection in it) even though it was invisible? And is this because he too was invsible?

I suppose it would be like a bubble of invisibility surrounding Perseus so that he could still see (all of?) himself and all his stuff but no one else could. The exception to this is perhaps Athena, who Apollodorus (in Bibliotheca 2.36-42) says guided Perseus' hand to strike the killing blow against Medusa while he was using the polished shield's reflection. The same account also seems to describe Perseus wearing the invisibility cap the entire time from before until after his encounter with the three Gorgons.

Are these the best clues as to whether everything borne upon one's person becomes invisible to others while wearing the "cap of darkness," or is there a clearer mention of this anywhere else?


Related: Does Medusa's head turn one's clothing to stone together with its wearer?

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In Homer's Illiad, in the conflict between Athena and Ares, Athena puts on the cap and becomes completely invisible to Ares.

Then Pallas Athene grasped the lash and the reins, and against Ares first she speedily drave the single-hooved horses. He was stripping of his armour huge Periphas that was far the best of the Aetolians, the glorious son of Ochesius. Him was blood-stained Ares stripping; but Athene [845] put on the cap of Hades, to the end that mighty Ares should not see her. Homers. Iliad. 5.835 from Perseus

From other works the Greek gods became "invisible" by being surrounded by a mist or cloud. This makes me think that the cap of Hades works in much the same way, and that by putting on the cap, Athena (for lack of a better word) doubles her invisibility and therefore becomes invisible to Ares while surrounded by mortals. Since Ares could see Athena on the battlefield without the cap, using the cap was a tactical advantage over Ares' brutishness and in par with what we know of Athena.

Donning the cap made Athena invisible to Ares when he confronted her favored warrior Diomedes. Ares on foot strikes first at Diomedes, who remains in his chariot with the goddess invisible beside him, the spear is deflected and Diomedes with the divine help of Athena wounds Ares, prompting Ares to return to Mt. Olympus to lick his wounds (and whine to Zeus about it prompting Zeus to call him the most spiteful of the gods).

This is noted in G. Kirk's 1985 The Iliad: A Commentary. Kirk notes :

844—5 The resumptive phrase τὸν μὲν “Ἄρης comes quickly, after only a Single v. of contextual detail, but is forceful in its repetition of the shocking ἐνάριζε and the Addition of μιαιφόνος, a standard epithet for the god but exemplified in drastic action here. It leads into Athene's unparalleled donning the cap of Hades, yet another of the exotic details for which this book is famous. Unlike others, ikhor for example, this has little to do with the special theme of wounding gods, and departs from the usual divine means of invisibility, namely covering with cloud or mist. The cap of invisibility, a widely diffused folktale concept, is enshrined in the Perseus myth (Apollodorus 2.4.2). which is especially rich in folktale motifs and devices; but this is naturally its earliest testimony in a Greek context, followed by Aspis 227. Its description here obviously draws on the popular etymology of Hades as ἀ-ξίδης the unseen one; that is emphasized by μή μιν ἴδοι and the repeated ὡς δὲ ἴδε of the next v. Perhaps the cloud mechanism seemed too unwieldy for a divinity in motion and with a mortal close beside her; moreover the idea of one deity joining a mortal in physical action against another highly untypical, one that might seem to call for untypical details in its description. The Iliad : a commentary by Kirk, Geoffrey Stephen p.158

Archive.org has a readable version and I can recommend putting it on your reading list.

From this we can be fairly certain that the cap confers complete invisibility, I think you are right in that Perseus was in a sort of bubble, in a way that he could still see himself and the items he was wearing while being invisible to others.

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    I think it's an awesome deep-dive of an Answer, actually, investigating the wacky mechanics of the issue.
    – Adinkra
    Jan 6 at 15:36
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    @Adinkra thank you. I'm happy to hear that! Do you feel it needs more to become a complete answer?
    – Tom Sol
    Jan 6 at 23:05
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    From the angle at which you're attacking the Question [to use the language of conflict, since we're talking about contending with monsters x-P ], I'd say that, near as I can tell, it is a complete Answer. (If it's fair to say, I'm also conflicted after seeing the Answer by @StephanMatthiesen , which I wish I could also Accept, in tandem with yours [since it's approaching the problem from a completely different angle, one I'd've never expected myself].)
    – Adinkra
    Jan 15 at 4:07
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    @Adinkra I totally agree with you. The bounty in his answer was well deserved. It is a very interesting point of view.
    – Tom Sol
    Jan 15 at 8:09
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+100

One can perhaps approach it from a perspective of the history of science and think about how the people at the time would have understood the physics of seeing and light.

You ask the very good question: "if the shield did become invisible because he was carrying it, how could he use it? Could he himself see it (and the reflection in it) even though it was invisible?"

First let's think about it in modern physics, and then the physics that ancient Greeks may have known.

With our knowledge of modern physics, this is indeed a problem and essentially wouldn't work. If there is any physical effect that would make you invisible, it would imply that you also can't see anything. To be invisible, all light will have to go through you without interacting, just like neutrinos fly through the Earth without interacting with the matter much. But then you can't see anything because the retina in your eye also wouldn't interact with the light, in the same way as it is very hard to detect neutrinos even though space is flooded with them (you need enormous detectors with huge mass to get the occasional interaction).

The other option would be to redirect the light around you with some convincing mirror or hologram system. Again you would need some opening to be able to see out, which implies that the opponent can also see in, although in practice these eyeholes could be small so that you're very hard to spot.

However, the modern understanding of how seeing works (light from some source being reflected by objects and detected by the eyes) was developed much later by Alhazen in the Book of Optics around 1010-1020 AD. In classical antiquity, there were two main theories: the emission theory and the intromission theory.

In the emission theory, seeing works essentially like a modern radar device: The eye sends out some kind of beam which then get sent back by the object. In the intromission theory, some physical representation from the object floats off to the eye, perhaps a bit like a TV broadcast. This was all a bit vague and there many problems with it, and there wasn't any understanding of what light rays were; this was imagined as some kind of mechanical interaction where the object sends out something like a little copy of itself (to put it simply).

In these theories, a stealth cap can work and you can still see out from underneath. In the emissions theory, it would work in a similar way as the modern "stealth" aircraft which are invisible on a radar, because they absorb or scatter the enemy's radar beam so it doesn't go back to their detector. But still the stealth aircraft can send out its own radar beams without any problem. So, in the emissions theory, the "eye beams" from Medusa's eyes get scattered by the cap, but the "eyes beams" from Perseus can still get out and he can see anything. In the intromission theory, the stealth cap can simply stop anything underneath from broadcasting its images.

It would perhaps work like a jamming transmitter, so it wouldn't matter what the person wears and that the sword and shield are large, their emissions would all just be jammed.

So if people tried to rationalise this, they may have argued along those lines. However, I would imagine that most people who wrote, retold or heard the stories, didn't really try to explain it in terms of the contemporary understanding of physics like today's science fiction fans do, and just accepted that this was some mythical device that worked in mysterious ways.

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    Very interesting way of looking at this question. Welcome to Mythology and Folklore!
    – Tom Sol
    Jan 6 at 20:55
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    Wow! Welcome indeed! I really appreciate the depth of consideration regarding perspectives (haha! literally & scientifically)!
    – Adinkra
    Jan 7 at 8:59

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