Mikvaos, Chapter 7, Mishnah 4. We are still dealing here with shinui mareh, the case of a mikveh whose water has taken on a different appearance. Earlier in the perek we learned the rule: should wine, or mohel (the liquid that comes out of pressed olives), or any similar fruit juice drop into a mikveh and tint the water so that it no longer looks like water, the mikveh is unfit as long as that tint remains. The question raised now is what the halachah is when the discoloration has spread over only a part of the mikveh and not over all of it.
The Mishnah says: "naflu lesocho yayin umohel veshinu miktzas mar'av." Wine or the liquid of olives fell into the mikveh and discolored only part of it. The tint has not yet spread through the whole body of water, so a portion of the mikveh still contains clear water, water that looks like water.
The Mishnah rules: "harei zeh lo yitbol bo", one may not immerse in such a mikveh, and the reason is that it does not hold forty se'ah possessing "marei mayim", the appearance of water. This holds true even if the person plans to go down specifically into the section that remained clear. Once the water that still looks like water falls short of the required measure, there is no kosher mikveh in front of him at all, and dipping in the untinted part achieves nothing.
So what can be done? The very same solutions apply that were given for a mikveh that turned discolored. Should the mikveh have held its complete forty se'ah before the wine or the olive liquid entered, one is permitted to add drawn water, even water brought in from the town's supply, continuing until the water once again looks the way water is supposed to look. But if forty se'ah had not yet accumulated there, that solution is unavailable to him. He has no choice but to wait until more rainwater comes in and completes the measure, thinning out the tint until the appearance of water is restored.
The lesson of this mishnah is that the forty se'ah of a mikveh must be forty se'ah of water in appearance as well as in substance. We do not measure only the section a person plans to immerse in. The mikveh is judged as a whole, and until the whole of it holds forty se'ah bearing the color of water, it is not yet a mikveh in which one may be toivel.