540 THE PRINCIPLES OF SCIENCE. [CHAP. 



All other predictions in optical science are, however, 

 thrown into the shade by the theoretical discovery of 

 conical refraction by the late Sir W. E. Hamilton, of 

 Dublin. In investigating the passage of light through 

 certain crystals, Hamilton found that Fresnel had slightly 

 misinterpreted his own formulae, and that, when rightly 

 understood, they indicated a phenomenon of a kind never 

 witnessed. A small ray of light sent into a crystal of 

 arragonite in a particular direction, becomes spread out 

 into an infinite number of rays, which form a hollow 

 cone within the crystal, and a hollow cylinder when 

 emerging from the opposite side. In another case, a 

 different, but equally strange, effect is produced, a ray of 

 light being spread out into a hollow cone at the point 

 where it quits the crystal. These phenomena are pecu- 

 liarly interesting, because cones and cylinders of light are 

 not produced in any other cases. They are opposed to all 

 analogy, and constitute singular exceptions, of a kind which 

 we shall afterwards consider more fully. Their strangeness 

 rendered them peculiarly fitted to test the truth of the 

 theory by which they were discovered ; and when Professor 

 Lloyd, at Hamilton's request, succeeded, after considerable 

 difficulty, in witnessing the new appearances, no further 

 doubt could remain of the validity of the wave theory 

 which we owe to Huyghens, Young, and Fresnel. 1 



Predictions from the Theory of Undulations, 



It is curious that the undulations of light, although in- 

 conceivably rapid and small, admit of more accurate mea- 

 surement than waves of any other kind. But so far as we 

 can carry out exact "experiments on other kinds of waves, 

 we find the phenomena of interference repeated, and 

 analogy gives considerable power of prediction. Herschel 

 was perhaps the first to suggest that two sounds might be 

 made to destroy each other by interference. 2 For if one- 

 half of a wave travelling through a tube could be sepa- 



1 Lloyd's Wave Theory, Part ii. pp. 5258. Babbage, Ninth 

 Bridgewater Treatise, p. 104, quoting Lloyd, Transactions of the 

 Royal Irish Academy, vol. xvii. Clifton, Quarterly Journal of Pure 

 and Applied Mathematics, January 1860. 



2 Encyclopaedia Metropolitana, art. Sound, p. 753. 



