EPOCH OF YOUNG AND FRESNEL. 443 



the phenomena. He moreover calculated the length 

 of an undulation from the measurements of fringes 

 of shadows, as he had done before from the colours 

 of thin plates ; and found a very close accordance 

 of the results of the various cases with one another. 



There is one difficulty, and one inaccuracy, in 

 Young's views at this period, which it may be pro- 

 per to note. The difficulty was, that he found it 

 necessary to suppose that light, when reflected at 

 a rarer medium, is retarded by half an undula- 

 tion. This assumption, though often urged at a 

 later period as an argument against the theory, was 

 fully justified as the mechanical principles of the 

 subject were unfolded ; and the necessity of it was 

 clear to Young from the first. On the strength 

 of this, says he, " I ventured to predict, that if the 

 reflections were of the same kind, made at the sur- 

 faces of a thin plate, of a density intermediate 

 between the densities of the mediums surrounding 

 it, the central spot would be white; and I have 

 now the pleasure of stating, that I have fully verified 

 this prediction by interposing a drop of oil of sas- 

 safras between a prism of flint-glass and a lens 

 of crown-glass." 



The inaccuracy of his calculations consisted in 

 his 'considering the external fringe of shadows to be 

 produced by the interference of a ray reflected from 

 the edge of the object, with a ray which passes clear 

 of it; instead of supposing all the parts of the wave 

 of light to corroborate or interfere with one another. 



