PHYSICS OF THE NINETEENTH CENTURA-LIGHT. 451 



rent film, and i P be a ray of light falling upon it at P, part of that 

 ray is reflected along P R and part enters the film in the direction P H, 

 and this arriving at H, is (wholly or in part) reflected in a ray along 

 H F, which emerges from the film in the direction F G parallel to P R. 

 Now, it is plain that the light which follows the course i p H F G has 

 to pursue a longer path than that which follows the course i p R, and 

 therefore it may happen that the undulations which have followed 

 the longer course shall be half a wave-length, or some odd multiple of 

 the half-wave-length, behind the undulations which have followed the 

 shorter course ; in such a case the rays would destroy each other, and 

 the result would be darkness. In this way the black rings seen in 

 Newton's experiment with monochromatic light are accounted for. 

 Ordinary light causes a transparent film to appear coloured when the 

 thickness of the film is such that the set of undulations on which some 

 particular colours depend are thus made to destroy each other by in- 

 terference. The film then appears tinted with the complementary 

 colour. Thus if it be the green rays which are destroyed, the film will 

 appear red, for this is the colour which predominates in the rays of 

 the spectrum remaining after abstraction of the green. Young gives a 

 very simple construction to show the conditions required in order that 

 a filni'may exhibit one and the same colour throughout, the result being 

 that the thickness of the film must vary as the secant (page 61) of the 

 angle of reflection r and this agrees exactly with Newton's experiments. 

 Another case of the interference of light was pointed out by Young 

 in the effect fine fibres have upon light. Coloured fringes are visible 

 when a candle is looked at across a single thread, as spun by the silk- 

 worm. The cause of these coloured fringes, said Young, must be 

 sought in the interference of the portions of light bending round each 

 side of the fibre. 



The next great discovery regarding light was made by a young 

 French mathematician, STEPHEN Louis MALUS (1775 1812). Malus 

 was one of the first pupils of the celebrated Ecole Poly technique, where 

 he soon acquired the friendship of Monge, the mathematician. He 

 entered the engineering corps of the French army at the age of twenty- 

 one, and was immediately engaged on active service in Flanders, Ger- 

 many, Egypt, and Syria. His experiences in the East included marches, 

 battles, sieges, sackings of towns, and an attack of the plague. In 

 1 80 1 he married, and in 1802 we find him engaged in military engi- 

 neering duties at Lisle, and he was subsequently employed in construct- 

 ing fortifications at Antwerp, Strasbourg, and Kehl. In 1807 Malus 

 presented his first paper to the Academy of Sciences, to the member- 

 ship of which he was elected in 1810. He died in 1812 at the early 

 age of thirty-seven. The discovery with which the name of Malus 

 will ever be associated is that of the polarization of light by reflection. 

 The phenomena of polarization are not only among the most curious 

 and brilliant in the whole range of optical science, but their study has 



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