EPOCH OF YOUNG ASD FKESXEL. 95 



There is one difficulty, and one inaccuracy, in Young's views at 

 this period, which it may be prefer to note. The difficulty was, 

 that he found it necessary to suppose that light, when reflected at a 

 raver medium, is retarded by half an undulation. 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 surfaces of a thin plate, 

 of a density intermediate between the densities of the mediums sur- 

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

 pleasure of stating, that I have fully verified this prediction by inter- 

 posing a drop of oil of sassafras 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 corrobo- 

 rate or interfere with one another. The mathematical treatment of 

 the question on the latter hypothesis was by no means easy. Young 

 was a mathematician of considerable power in the solution of the pro- 

 blems which came before him : though his methods possessed none of 

 the analytical elegance which, in his time, had become general in 

 France. But it does not appear that he ever solved the problem of 

 undulations as applied to fringes, with its true conditions. He did, 

 however, rectify his conceptions of the nature of the interference ; 

 and we may add, that the numerical error of the consequences of the 

 defective hypothesis is not such as to prevent their confirming the 

 undulatory theory. 2 



But though this theory was thus so powerfully recommended by 

 experiment and calculation, it met wilh little favor in the 'scientific 

 world. Perhaps this will be in some measure accounted for, when we 

 come, in the next chapter, to speak of the mode of its reception by 



2 I may mention, in addition to the applications which Young made of the 

 principle of interferences, his Eriometcr, an instrument invented for the pur- 

 pose of measuring the thickness of the fibres of wood; and the explanation of 

 the supernumerary bands of the rainbow. These explanations involve calcu- 

 lations founded on the length of an undulation of light, and were confirmed by 

 experiment, as far as experiment went. 



