Among those, whose candour and judgment I estimate highly, I distinctly remember six who expressed their objections to the Lyrical Ballads almost in the same words, and altogether to the same purport, at the same time admitting, that several of the poems had given them great pleasure; and, strange as it might seem, the composition which one cited as execrable, another quoted as his favourite. I am indeed convinced in my own mind, that could the same experiment have been tried with these volumes, as was made in the well known story of the picture, the result would have been the same; the parts which had been covered by black spots on the one day, would be found equally albo lapide notatæ on the succeeding.Well, this is Leask says: 'the "well known story" is untraced, but Coleridge refers to the Roman use of white and black stones to mark favourable and unfavourable days.' No, Coleridge doesn't. It's true that lapis means 'stone', but it also means a landmark, a mark or a spot: 'albo lapide notatæ' means, in this context, ‘distinguished by white spots’ (as opposed to the black spots mentioned earlier).
In fact, Coleridge does not mean a specific picture; he is referring to recent developments in the science of retinal optics. He may for instance have read the entry on ‘Retention’ in Nicholson’s British Encyclopedia: ‘Place about half an inch square of white paper on a black hat, and looking steadily on the centre of it for a minute, remove your eyes to a sheet of white paper ; after a second or two a dark square will be seen on the white paper, which will he seen for some time. … Again, make with ink, on white paper, a very black spot, about half an inch in diameter, with a tail about an inch in length, so as to represent a tadpole. Look steadily at this spot for about a minute, and on moving the eye a little, the figure of the tadpole will be seen on the white part of the paper, which figure will appear whiter or more luminous than the other part of the paper. This Dr. R. Darwin brings as one proof, that when the retina has been subjected to a less excitement, it is more easily brought into action by being subjected to a greater. A surface appears black in consequence of its absorbing all the rays of light; that part of the retina, therefore, which is unemployed while looking at the spot, is afterwards more sensible of the light from the white paper. than those parts which had previously been exposed to it.’ [‘Retention’, William Nicholson, British Encylopedia (6 vols, 1809), 5:450].