STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 



these particularly, we find that each of them is blue on 

 the side nearest to the true image, and red on the more 

 distant. Our investigation will be simplified if we con- 

 sider the action of a single eyelash. We can then reason 

 from that to the conjoint action of all. 



It is necessary, however, in the first place, to recall 

 some facts connected with the wave theory of light. 

 The foundations of this theory were laid by Huyghens, 

 the great Dutch philosopher, contemporary with Newton, 

 but its construction advanced very slowly, being opposed 

 by the great authority of Newton, who favored the cor- 

 puscular or emission theory, and regarded light as con- 

 sisting of particles emitted with excessive velocity from 

 shining bodies. Although there were facts, such as those 

 connected with double refraction, easily accounted for 

 by the system of undulations, but inexplicable on the 

 emission theory, these were put aside, in the expectation 

 that they would in the course of time be successfully 

 dealt with. It was not until the publication of a 

 course of lectures on natural and experimental philos- 

 ophy by Dr. Young in 1802, in which he announced the 

 great discovery of the interference of light, that the un- 

 dulatory theory could no longer be overlooked. This 

 discovery was, however, still ridiculed by the Edinburgh 

 Review, and Young's explanations so bitterly attacked 

 that he was constrained to publish a pamphlet in reply. 

 Of this it is said that only a single copy was sold. 



In 1819, a memoir by Fresnel was crowned by the 

 French Academy of Sciences. He discovered that the 

 vibratory movements in the ether constituting light are 

 transverse to the course of the ray. His views are em- 

 bodied in what is now known as the theory of trans- 

 verse vibrations. 



The conflict between the rival theories was eventually 

 settled by the experiments of Fizeau and Foucault. On 



