2iS PROGRESS OF SCIENCE. 



the other, so that a ray was divided into two waves moving at 

 different rates through the crystal. From experiments bear- 

 ing upon this fact, he discovered another phenomenon called 

 POLARISATION of light the effects of which exhibit "the most 

 gorgeous colours in the whole domain of optics." The ex- 

 planation of this phenomenon we owe to the beautiful analysis 

 -of Fresnel. But, as we see, Huygens anticipated the discoveries 

 of Malus, Fresnel, and Young, though he failed to demonstrate 

 their physical causes as they wer,e demonstrated by these 

 three physicists. The main fact, however, remains untouched : 

 he may be said to have understood and explained the un- 

 dulatory theory. But as Newton had formulated a different 

 theory, and his authority in science was held to be supreme 

 in all matters, Huygens's labours were laid aside until the 

 beginning of this century, when it was proved he had been 

 right all along the line. 



1638 1703. Hooke applied a SPIRAL SPRING to the 

 balance OF A WATCH in 1658; invented the dial barometer; 

 the octant, subsequently improved into the sextant ; improved 

 the microscope; discovered THE PRINCIPLE OF INTER- 

 FERENCE a principle of great importance in optics. (See 

 Grimaldi and Young.) He expressed his belief in the 

 undulatory theory of light, and with equally rare insight 

 determined the essential conditions of combustion just about 

 the same time as Boyle and Mayow. 



1642 1727. Newton, as a physicist, enunciated the 

 corpuscular (or emissive) theory of light (1666), in opposition 

 to the undulatory theory of Kepler, Descartes, Huygens, and 

 Hooke. Newton believed that light consists of an emanation 

 of exceedingly minute particles of matter, projected from the 

 sun and other luminous sources with enormous velocity. 

 Modern science has settled the point. For once Newton 

 proved to be wrong. Optics are, nevertheless, deeply 

 indebted to his marvellous genius for his third great 

 discovery : he found out the composition of white light 

 by the same means as Descartes THE PRISM : in other 

 words he DECOMPOSED WHITE LIGHT into the prismatic 

 colours, which he called SPECTRUM, a word to be re- 

 membered ; and then recomposed white light, by the means 



