The issue arose because of the following question: The Gemara says that you cannot make the bracha on a fire that was lit on Shabbos, so it must be made on a fire in the home or the hands of a Jew, someone who observes the halachos of Shabbos. If you are passing a town and see a fire, if half the residents are Jews, you are allowed to assume it was lit by a Jew and you may make the bracha. If the majority of residents are Gentiles, you cannot make the bracha. The problem is that this is a classic example of קבוע, a case where the objects in the mixture are stationary. The halacha is that כל קבוע כמחצה על מחצה דמי, a קבוע mixture is legally viewed as fifty fifty, no matter what the actual numbers are. A fire in an oven is a perfect case of קבוע, and the ratio of Jew to Gentile should be irrelevant. Even if the majority are Gentiles, it should have the legal status of fifty fifty and it should be muttar to make a bracha on the light.
The Magen Avraham asks this question in 298 SK 13. He anticipates and refutes a possible answer by saying that the Gemara does not appear to be discussing a case of a torch in a walking person's hand, but rather a fire within a home. So he answers that light, and the fragrance of besamim, is considered Piresh, something that has left its place of origin. (I don't know the history of the scientific concept, but for the Magen Avraham to assume that light travels is interesting.)
I find this answer difficult. (The next section was edited in response to a comment from Chaim B.)
First, I think that the light and its source should be viewed, in halacha, as one thing, as if the light is an an expansion of the source rather than something being propelled from the source. So if the source is kavua, the light has a din kavua too.
Second, we're not paskening on the light, we're deciding whether the source is ra'ui to make a bracha on it. Even if we're making the bracha on the light that reaches us, it's only on the basis of what we decide is the nature of the source. It doesn't make sense to take off the rule of kavua from the source that we're paskening on because the light that came to us is piresh.(In other words, the basic din of Kavua applies to the case of זורק אבן לגו (Kesuvos 15a). I think that it applies equally to a case of אחד מן הגו שזרק את האבן, but the Magen Avraham obviously holds that it does not.)
Third, I don't believe we're making the bracha on the light that is reaching us. We're making the bracha on the source itself. Yes, you know that the torch is there on the basis of the light that reaches your eyes. But the bracha is on the torch, not on the light that reaches you. In the case of Besamim, I understand the Magen Avraham. Besamim is a birkas hanehenin; you are making the bracha on your enjoyment of the fragrance. That certainly is on the smell that comes to you. True, you do not make a bracha on a smell that has no tangible source, but that's only a matter of chashivus, and the bracha is on the fragrance that reaches you. But the bracha on fire is birkas שבח והודאה, it is a bracha of praise and appreciation. We are expressing appreciation for the fire itself, not the light that reaches us.
The person that asked the question suggested a slight modification of the answer the Magen Avraham rejected. This might be a case where we are not sure what the source of the light is, whether it stationary or being carried in the street. If it is stationary, it is קבוע; if it is moving, it is פרוש. If it is פרוש, we would follow the majority and assume it is of a gentile. If it is קבוע, it would be viewed as fifty fifty. In such a case, the logical result is that you still have a majority: If קבוע, then fifty fifty; if פרוש, then sixty forty. So the total is still fifty five forty five.
This happens to be a machlokes in Kiddushin 73, between the Rashba/Ritva, who view ספק קבוע as a רוב, and the Pnei Yehoshua there, who views it as no different than a regular קבוע. At least according to the Rashba and Ritva we can suggest this pshat in the Gemara.
My son in law pointed out that the Magen Avraham is not the only one to say this svara- it is also well known from the Mordechai, quoting his Rebbi, Rabbeinu Yakar, in Chulin 11, where he asks, how can we learn the din of rov by certain cases of Rov from Sanhedrin, when Sanhedrin is a classic example of kavua. He says two answers, and the second is דאין עושין בגופן אלא בדיבור הנבדל והנפרש מפיהם והוי כבשר הנמצא דאמרי' בי' כל דפריש מרובא פריש , which means that when we pasken like the majority of Sanhedrin, we're not paskening on the bodies of the dayanim, we're paskening on their oral expressions of opinion, and that oral expression is not kavua, it's parush. This is exactly like the Magen Avraham, and is equally incomprehensible, as Reb Naftali Trop hints in his comment on the Reb Chaim in the stencils number 229, because, as my brother put it, according the Rabbeinu Yakar's approach, if you have a kavua of nineteen goyim and one Jew, you can say Kadish because you have a minyan.
Here's the entire Mordechai:
I later found that the connection of the Magen Avraham to the Mordchai is brought in the Maharitz Chiyos/Chajes in the sugya of Me'orei Ha'eish in Brachos 53a. Eli pointed out that it is also noted in the Machatzis Hashekel and the Reb Akiva Eiger in Shulchan Aruch there.