EXCEPTIONAL PHEXOMEXA. 319 



passing through certain double-refracting crystals. The 

 laws obeyed by the wave are exactly the same as in other 

 cases, yet the results are entirely sui generis. So far are 

 such cases from contradicting the theory of ordinary 

 cases, that they afford the supreme opportunities for 

 verification. 



In astronomy singular exceptions might occur, and in 

 an approximate manner they do occur. We might point 

 to the rings of Saturn as objects which, though undoubt- 

 edly obeying the law of gravity, are yet entirely unique, 

 as far as our observation of the universe has gone. They 

 agree, indeed, with the other bodies of the planetary 

 system in the stability of their movements, which never 

 diverge far from the mean position. But a truly singular 

 event might happen, or might have happened, under 

 slightly different circumstances. Had the rings been 

 exactly uniform all round, and with a centre of gravity 

 coinciding for a moment with that of Saturn, a singular case 

 of unstable equilibrium would have arisen, necessarily re- 

 sulting in the sudden collapse of the rings, and the fall of 

 their debris upon the surface of the planet. Thus in one 

 single case the theory of gravity would give a result 

 wholly unlike anything else known in the mechanism of 

 the heavens. 



It is possible that we might meet with singular excep- 

 tions in crystallography. If a crystal of the second 

 or dimetric system, in which the third axis is usually 

 unequal to either of the other two, happened to have the 

 three axes equal, it might be mistaken at first sight for a 

 crystal of the cubic system, but would in many ways 

 exhibit different faces and dissimilar properties. There 

 is, again, a possible class of diclinic crystals in wliich two 

 axes are at right angles and the third axis inclined to the 

 other two. This class is chiefly remarkable for its non- 

 existence in a material point of view, since no crystals 



