A VOYAGE IN SPACE
slow curvature. Even now you could not distinguish the real Uranus from the theoretical one in the calculated place. The disturbance can only be seen when that again is magnified 50 times; then you can just see the difference between the real and expected places. That small difference in the picture, diminished 2500 times, was all that Adams and Leverrier had to go upon, but from that tiny discrepancy they were able to infer the existence of the mighty planet Neptune.
Another point worthy of notice is that Bode's Law had by this time taken so firm a place in men's minds that both Adams and Leverrier used it to help them in their calculations. This is scarcely surprising when we remember that, first of all, Uranus had been found to fit in with the law; and that, secondly, the gap had been filled by the minor planets. But unfortunately this law, which they thought would be a help, was only a hindrance, for it no longer held. You may at some time or other have been going down a dark staircase, perhaps in some old tower, and the steps have been so even for a long time that you think you know just how far down to put your foot for the next, when suddenly there comes a short step or an extra long step which gives you quite a shock. It is often like that in scientific work: you think you have found out some law which will enable you to set your foot confidently on the next step, but owing to some unknown cause the next step is of a different length and you get a shock. Sometimes you can find out the reason for the exception, which soon leads to a discovery; sometimes the reason is not found for a long time. We do not