All can understand how frightened the bonde became when he saw that Thórr let his brows sink down over his eyes. When he saw his eyes he thought he must fall down at the sight of them alone.
Prose Edda verse 45
Did you ever wonder why it is that many Thor’s Hammer symbols have eyes on the upper part? As you can read in the quote that opens this short article, there is something about those eyes of Thor. The sight of them alone makes the farmer whose son Thialfi broke one of the bones of one of Thor’s goats, think that he will faint. The only description that you get here is that Thor’s brows “sink down over his eyes”. Here we have a point that did not pass entirely unnoticed. Let me quote Georges Dumézil at length as he describes and compares similar events in different sources in his The Destiny Of The Warrior (p. 161/2/3):
This sign [i.e. the piece of the whetstone that got stuck in Thor’s head after his fight with the giant Hrungir], a consequence of the god’s victory in his first einvígr, recalls one of the signs -numerous, excessive, often monstrous- which appear upon the young Cúchulainn after his first combat. Some of these immediately become stable features, others reappear only in the hero’s attacks of martial furor. The sign that is similar to þhórr’s is mentioned in the episode in the Macgnímrada of the Táin Bó Cuailnge, “rising from the summit of his skull.” In the In carpat serda episode, however, it is described with much greater precision: ‘The moon of the hero protruded from his forehead, as long, as thick, as the whetstone of a warrior, as long as the nose.” Some of the figures represented on certain Gallic coins also have an emanation protruding from the forehead, sometimes in the form of a round-headed nail. It is likely that they attest to an appreciated by the contental Celts of the same stigmata of valor.
Among the “forms” which appear on the victorious Cúchulainn, the greatest number, I repeat, are fantastic. There are some, however, which may be no more than exaggerations of a heroic grimace. A remarkable example is to be found in the Macgnímrada episode: “He closed one of his eyes to the point where it was no larger than the eye of a needle, and he opened the other wide to the point where it was as big as a cup of mead”; and in In carpat serda: “He swallowed one of his eyes in his head, to the point where a wild heron would have had trouble managing to bring it from the bottom of his skull to the surface of his cheek, while the other jutted out and sat on his cheek, on the outside.” Without permitting themselves such distortions, the Viking adventurers, when it came to solemn circumstances, assumed attitudes and made grimaces of their own which established their rank and dignity, and backed up their demands. Behind such countenances there was probably an older tradition. Received and fully banqueted by king Aðalstein, from whom he has the right to expect a large remuneration, Egill, the warrior scald, sits down on the far side of the hall, on the seat of honor, facing the king. He keeps his helmet on his head, puts his shield at his feet and his sword on his knees, alternately drawing the sword halfway out and putting it back in its scabbard. He holds himself stiff and straight, and refuses all drink. In addition, again alternately, he makes one of his eyebrows drop down to his chin while lifting the other to the hairline. The effect must have been impressive, for he had contigious eyebrows above his black eyes. The king then gets up, puts a highly precious ring on the tip of his bare sword, walks toward the Viking and offers him his present over the hearth. The Viking gets up in turn, sword bared, approaches the hearth from the other side, and receives the ring on the tip of his sword. Both sit down again. Egill puts the ring on his finger. And only then do his eyebrows return to their normal position. He lays down his sword and shield and accepts the cup, which until then has been offered in vain.
Especially when put together like this, these scenes may seem very amusing, but on the other hand, they also suggest that they are far from being so. Once you noticed these weird events, you will actually find out that they can be found both in Celtic stories and in Icelandic sagas and Norse tales quite frequently. Dumézil simply puts a few examples together in a few passages in different books, but he does not really explain his ideas, but his great terms “martial furor” and “stigmata of valor” say pretty much. After the text that I quoted, he gives a few examples from Indian mythology which refer to the same phenomenon. He closes the appendix (and the book) with the words: “But could it not be simply the stylization into laksana, into a congenital sign, of a “form,” a delb in the Irish manner, appearing on the tested warrior and distinguishing his appearance from that of the ordinary man, a “form,” which, in its origin, is probably derived from a traditional heroic contortion?”
One thing is very clear: it is all about warriors and since Thor fills the warrior-function in the Dumézilian scheme, so I don’t it is strange to connect he passages about his fiery eyes to the odd faces the Germanic and Celtic warrior pull on certain occasions. I do not fully agree with the comparison of the whetstone and the faces, but Dumézil makes an interesting point and the stone in Thor’s head is by some people referred to as his third eye. When you think of it, there is quite some focus on the warrior’s eyes so my suggestion is that the eyes on Thor’s hammer may refer to that as well. More even, when you compare the two examples that I give you, one may even be a ‘good news’ hammer (eyes wide open) and the other a ‘bad news’ hammer (brows dropped)! This all is of course hardly enough to draw conclusions, but all in all it makes a nice suggestion and a possible explanation for the eyes.