Precisely as in such a case as we have just considered, if we were obliged to test the water by so inexact a method, we should make experiments with one hand only, and carefully consider the condition of that hand during the progress of the experiments, so in the case of the floor of Plato, we must exclude as far as possible all effects due to mere contrast. We must examine the tint of the plain, at lunar morning, mid-day, and evening, with an eye not affected either by the darkness or brightness of adjacent regions, or adjacent parts of the same region. This is very readily done. All we have to do is to reduce the telescopic field of view to such an extent that, instead of the whole floor, only a small portion can be seen. It will then be found, as I can myself certify (the more apparently because the experience of others confirms my own), that the supposed change of tint does not take place. One or two who were and are strong believers in the reality of the change do indeed assert that they have tried this experiment, and have obtained an entirely different result. But this may fairly be regarded as showing how apt an observer is to be self-deceived when he is entirely persuaded of the truth of some favourite theory. For those who carried out the experiment successfully had no views one way or the other; those only failed who were certainly assured beforehand that the experiment would confirm their theory. The case of the lunar crater Linne, which somewhere about November 1865 attracted the attention of astronomers, belongs to a very different category. In my article on the moon in the "Contemporary Review" I have fully presented the evidence in the case of this remarkable object. I need not therefore consider here the various arguments which have been urged for and against the occurrence of change. I may mention, however, that, in my anxiety to do full justice to the theory that change has really occurred, I took Maedler's description of the crater's interior as "very deep," to mean more than Maedler probably intended. There is now a depression several hundred yards in depth. If Maedler's description be interpreted, as I interpreted it for the occasion in the above article, to mean a depth of two or three miles, it is of course certain that there has been a very remarkable change. But some of the observers who have devoted themselves utterly, it would seem, to the lively occupation of measuring, counting, and describing the tens of thousands of lunar craters already known, assert that Maedler and Lohrman (who uses the same description) meant nothing like so great a depth. Probably Maedler only meant about half a mile, or even less. In this case their favourite theory no longer seems so strongly supported by the evidence. In some old drawings by the well-known observer Schroeter, the crater is drawn very much as it now appears. Thus, I think we must adopt as most probable the opinion which is, I see, advanced by Prof.