264 The Evolution of Optics. 



one human mind ; its wide-reaching influence, the heritage 

 of intellectual wealth which one man may bequeath to all 

 who come after him. The failures, too, are as instructive as 

 the greatest triumphs. "What more eloquent protest against 

 the domination of authority in scientific inquiry could be 

 found than is contained in this chapter of the history of 

 optics? The weight of Newton's authority served to up- 

 hold almost to our own day the corpuscular theory of light. 

 It blocked paths of investigation which led to very treasure- 

 houses of discovery. His masterful intellect impressed with 

 equal force error and truth. 



Newton's great contribution to the science of optics was 

 his demonstration of the composite nature of light. He 

 showed white light to be a mixture of different kinds of 

 light of varying refrangibility ; he made his demonstration 

 conclusive by analyzing and then recombining the constitu- 

 ents. To explain the phenomena of light, Newton con- 

 structed the emission theory. Conceiving light to consist 

 of small particles shot out from luminous bodies, he ex- 

 plained the phenomena of reflection by elastic collision, and 

 refraction by an attractive force similar to that of gravitation. 



This theory Newton developed with great ingenuity, and 

 by it many of the phenomena of light were seemingly ex- 

 plained; where it fell short, Newton supported it with a 

 weight of argument which for a time successfully bore 

 down all opposition. But new discoveries were constantly 

 adding difficulties to the acceptance of this theory. In 1676 

 Bonier had proved, from observations of the eclipses of 

 Jupiter's satellites, that light traveled with the velocity of 

 some' one hundred and ninety thousand miles a second. 

 Now, it was very difficult to conceive that matter, however 

 minute, traveling with this enormous velocity, could be re- 

 ceived without injury upon the delicate tissues of the retina. 

 A rival theory of light, destined in time to supplant the 

 emission theory, had already been proposed. In 16G4 Hooke 

 had suggested, as a happy guess, the proposition that light 

 might consist of undulations in a homogeneous medium, and 

 a little later Huygens had developed this theory in a very 

 masterly manner, and showed that it offered a much more 

 rational explanation of light than did the emission theory ; 

 he investigated the phenomena of double refraction, first 

 observed by Bartholinus in 1669, and showed that this could 

 be explained by the wave-theory. In fact, Huygens seems 

 to have possessed a very accurate conception of the nature 



