160 DISCOVERY OH. 



the inch, and that the number of such waves in a given 

 length increases progressively in passing from red to 

 violet, until at this end of the colour scale there are 

 about 60,000 undulations to the inch. Young also 

 proved that certain optical effects could be explained 

 only by the principle of interference of ether- waves with 

 one another ; but his researches and interpretations, 

 involving as they did the existence of an imponderable 

 ether which, to use his words " pervades the substance 

 of all material bodies, with little or no resistance, as 

 freely, perhaps, as the wind passes through a grove of 

 trees," met with ridicule from leaders in the literary 

 world, and were not given serious attention by his 

 scientific contemporaries. When in 1815 a French 

 investigator, Augustin Jean Fresnel, began experimental 

 work in optics, and was also led to the discovery of 

 interference in light, he knew nothing of the previous 

 work done by Young in the same direction thirteen 

 years earlier. The work of these two investigators 

 revived the undulatory theory and opened a question 

 which may be said not to have been settled decisively 

 until Foucault's crucial experiment had been made. 



Here, then, we have an instance of a scientific theory 

 being abandoned when it had been shown to be untenable 

 by decisive experiment. It is possible that, as knowledge 

 increases, the theory which has held its own for three- 

 quarters of a century, may have to be modified ; indeed, 

 researches in the domain of radioactivity have proved 

 the existence of minute particles moving with the high 

 velocities demanded by Newton's corpuscular theory 

 of light. It has yet to be shown, however, that such 

 particles can account for optical phenomena as com- 

 pletely as the wave-theory does, though they are probably 

 concerned with radiation of all kinds. 



