It is Vienna, 1894, and I am painting a goldfinch; but even in this fantasy I am painting very badly. Still, I am a prince and so everyone is pretending that I am not too bad at it. My painting is complimented, and hung up in a corridor.
Eventually the painting is found by the Nazis, who take it for a bad painting of a robin. Since it would make Hitler's paintings look good, they move it to Berlin. Everyone ignores it though. They are very busy. After the war it ends up in Boston, where someone says: "That bird was very well fed." And it was a rather plump goldfinch. But these people imagine that they are talking about a robin. Some of them assume that it was an imaginary robin, it was so badly painted. But in fact, all of them were talking about a goldfinch.
Or, do you think that they were talking about robins? How very modern! To see why they were not, suppose that we are looking through a warped and dirty window, at a fat goldfinch in bad light. You might at first think that it is a robin. But you are looking at a goldfinch. If you point and say "that bird" then you are referring to a goldfinch. Were you a mad man, used to seeing things that are not there, you might assume that it was your own personal hallucination of a robin that you were seeing. But even then, your "that bird" would refer to the bird that you were looking at. That is just the way reference works in our public language. And it is clearly the same if we are looking, instead, at a very bad photograph, of a very fat goldfinch.
Suppose the photograph is so bad that someone takes it for a bad picture of a robin. When she talks about the bird in the picture, she is referring to something that was photographed. And even if she thinks that the picture is a sketch of an imaginary robin, it is still actually a photograph of a goldfinch. And we developed language on top of whatever else we already had, and so the same underlying principle would apply to the semantics of names; primarily to spoken names, and secondarily to written names like these: Mill knew that it did, whereas Russell appears not to have known.