6. It may be asked, How are we to decide in such alternatives? How are we to select the one right idea out of several conceivable ones? To which we can only reply, that this must be done by _trying_ which will succeed. If there really exist a cycle of the weather, as well as of eclipses, this must be established by comparing the asserted cycle with a good register of the seasons, of sufficient extent. Or if the moon really influence the meteorological conditions of the air, the asserted influence must be compared with the observed facts, and so accepted or rejected. When Hipparchus had observed the increase of longitude of the stars, the idea of a motion of the celestial sphere suggested itself as the explanation of the change; but this thought was _verified_ only by observing several stars. It was conceivable that each star should have an independent motion, governed by time only, or by other circumstances, instead of being regulated by its place in the sphere; and this possibility could be rejected by trial alone. In like manner, the original opinion of the composition of bodies supposed the compounds to derive their properties from the elements according to the law of _likeness_; but this opinion was overturned by a thousand facts; and thus the really applicable Idea of Chemical Composition was introduced in modern times. In what has already been said on the History of Ideas, we have seen how each science was in a state {192} of confusion and darkness till the right idea was introduced. 7. No general method of evolving such ideas can be given. Such events appear to result from a peculiar sagacity and felicity of mind;--never without labour, never without preparation;--yet with no constant dependence upon preparation, or upon labour, or even entirely upon personal endowments. Newton explained the colours which refraction produces, by referring each colour to a peculiar _angle of refraction_, thus introducing the right idea. But when the same philosopher tried to explain the colours produced by diffraction, he erred, by attempting to apply the same idea, (_the course of a single ray_,) instead of applying the truer idea, of the _interference of two rays_. Newton gave a wrong rule for the double refraction of Iceland spar, by making the refraction depend on the _edges_ of the rhombohedron: Huyghens, more happy, introduced the idea of the _axis of symmetry_ of the solid, and thus was able to give the true law of the phenomena.