KINETIC OR MECHANICAL VIEW OF NATURE. 23 



have unequal properties in different directions ; and the 

 process of revealing it was termed polarisation. Huygens 

 had discovered this property, which he found was given 

 to rays of light if they passed through certain crystals, 

 notably through Iceland spar, which has the capacity 

 of dividing the rays so that objects seen through them 

 appear double. He could not explain it on his 

 hypothesis of undulations, though he had invented a geo- 

 metrical construction of the double refraction which had 

 led him to its discovery. Malus showed in 1808 that 

 double refraction was not a necessary accompaniment of 

 polarisation, but that ordinary reflexion was enough to 

 give these sides to rays of light. Although the projectile 

 theory gave no complete explanation of this property, 

 still the supposition that this one- or many-sidedness 

 was owing to certain geometrical shapes of the pro- 

 jected particles suggested that double refraction might 

 be explained by the different attraction or repulsion 

 which these particles suffered according to the aspect 



determining the course of the ordi- 

 nary and extraordinary rays in Ice- 

 land .spar, described the pheno- 

 menon fully, admitting at the same 

 time that he could not explain it. 

 When Malus discovered that light 

 might acquire this peculiar pro- 

 perty by reflexion, Young wrote 

 in a review ( ' Quarterly Review,' 

 May 1810) : " The discovery . . . 

 appears to us to be by far the most 

 important and interesting that has 

 been made in France, concerning 

 the properties of light, at least since 

 the time of Huygens ; and it is so 

 much the more deserving of notice, 

 as it greatly influences the general 

 balance of evidence in the com- 

 parison of the undulatory and the 

 projectile theories of the nature of 



light " (Works, vol. i. p. 247). And 

 Malus himself, in writing to Young 

 as Foreign Secretary of the Royal 

 Society, by whom he had been 

 awarded the Rumford Medal, says : 

 " Je ne regarde pas la connaissance 

 de ces phe"nomenes comme plus 

 favorable au systerne de remission 

 qu'a celui des ondulations. Us d^- 

 montrent egalement I'insuffisance 

 des deux hypotheses ; en effet com- 

 ment expliquer dans 1'une ou <laus 

 1'autre pourquoi un rayon polarise" 

 peut traverser sous une certaine 

 inclinaison un corps diaphane, en 

 se derobant totalement a la re- 

 flexion partielle qui a lieu a la sur- 

 face de ces corps dans les cas ordi- 

 naires ? " (quoted by Peacock, ' Life 

 of Young,' p. 243 note). 



