Shabbos, Chapter 2, Mishnah 4. This mishnah again deals with the laws of lighting Shabbos candles, and specifically with the concern that a person might take some of the fuel from a lamp or a lantern that is burning. Taking fuel from a lamp involves the Torah prohibition of extinguishing.
The nature of the concern:
As a rule, we are not concerned that a person will take fuel on Shabbos. The concern exists only when the fuel is located outside the lamp, outside the vessel itself. In such a case a person might view it as something that is not part of the lamp and not part of the fuel currently burning, and think that he may take from it for some other purpose - for example, someone who lights with olive oil and wants a bit of oil for his own use. But since the fuel was designated for the lamp, taking it involves the prohibition of extinguishing. When the fuel is placed in the lamp itself, however, we assume that a person will realize that it is part of the lamp and will not take from it.
The wording of the mishnah:
"Lo yikov adam shefoferes shel beitzah vimalenah shemen v'yitnenah al pi haner bishvil shet'hei menateffes" - one may not pierce a hole in an eggshell, fill it with oil and place it above the mouth of the lamp so that it will drip into it and supply it with fuel.
"Afilu hi shel cheres" - even if instead of an eggshell a person uses an earthenware vessel. Earthenware becomes repulsive and off-putting once it has been used for oil and for burning, so seemingly there is less reason to think that a person would take fuel from it for eating. Nevertheless it was forbidden, out of concern that he might take from the oil for other purposes and thereby transgress the prohibition of extinguishing on Shabbos.
"V'Rabbi Yehudah matir" - Rabbi Yehudah is not concerned about this. In his view a person will recognize that the fuel dripping into the lamp is designated for that lamp, and will know that it is forbidden to take oil from it.
"Aval im chiber hayotzer mitechilah - mutar" - if the potter who made the lamp attached the oil reservoir to the lamp from the outset, it is permitted, even according to the first Tanna who forbids an eggshell or another vessel that drips into the lamp. For they are joined as a single vessel, and just as with an ordinary lamp we are not concerned that a person will take from the oil in the lamp, so too we are not concerned that he will take from the outer reservoir.
A bowl beside the lamp:
Similarly, a person may not put oil in a bowl, place it beside the lamp, and put the other end of the wick into it, so that the wick will draw not only from the oil in the lamp but also from the oil in the bowl. The reason for the prohibition is as explained: according to the first Tanna we are concerned that a person will think that the oil in the bowl is not designated for the lamp and is not considered part of it, and will think that he may take from it on Shabbos - whereas in truth this oil too, having been designated as subordinate to the lamp, may not be taken on Shabbos. Because of this concern, we do not permit the arrangement in the first place.
Here too Rabbi Yehudah permits it, because he is not concerned about this: in his view a person will recognize that the oil in the bowl is designated for the lamp, and will know that it may not be taken on Shabbos.
In summary: in this mishnah we learned that the concern for the prohibition of extinguishing exists specifically when the fuel reservoir is outside the lamp. The first Tanna therefore forbade an eggshell or an earthenware vessel that drips into the lamp, as well as a bowl of oil from which the wick draws; but when the potter attached the reservoir to the lamp from the outset, they are a single vessel and it is permitted. Rabbi Yehudah permits all of these, since he is not concerned that a person will fail to recognize that the oil is designated for the lamp and may not be taken.