Memoir of Dr. Thomas Young. 323 



for each species of simple colour there exists in thin plates 

 of every kind, a series of thicknesses increasing where no 

 light is reflected. This was a capital result. It contained 

 the key to all the phenomena. 



Newton was less happy in the theoretical views which 

 this remarkahle observation suggested to him. To talk 

 with him of a reflected ray of light being in a fit of easy 

 reflexion, or of a ray which passes completely through a 

 plate being in a fit of easy transmission, is nothing else than 

 to announce in obscure terms, what the experiment with 

 the two lenses had taught us. The theory of Thomas Young 

 escapes this criticism. Here, no fit of any kind is admitted 

 as a primary property of rays. The thin plate is found 

 always in a similar condition to a thick mirror of the same 

 substance. If, in certain points, no light was seen, Young 

 did not conclude that reflexion had ceased there ; he sup- 

 posed that in certain directions from these points, the rays 

 reflected by the second face in proceeding to meet the rays 

 reflected by the first are completely annihilated. It is to 

 this conflict of rays that the author gave the famous term 

 of interference. 



Certainly this is a remarkable hypothesis. It occasions 

 great surprize to find the night in the blaze of sunshine, in 

 points where the rays of this body are freely sent ; but who 

 would have imagined that darkness could be produced by 

 adding light to light. 



A natural philosopher is justly in a glorious position, 

 when he can announce some result in direct opposition to 

 the commonly received opinions ; but it is necessary with- 

 out delay to bring forward demonstrative proofs, otherwise 

 we shall be likened to oriental writers, whose reveries 

 amused the sultan Schahriar for a thousand and one nights. 

 Young was not so prudent. He shewed, first, how his 

 theory was adapted to the phenomena, but without pro- 

 ceeding beyond probabilities. When, subsequently, he ob- 

 tained true proofs, the public had objections which he 

 could not overcome. However, the experiment upon which 

 Young made his discovery rest, would not admit of the 

 shadow of a doubt. 



Two rays coming from the same source, and proceeding 

 by slightlv unequal routes crossed each other at a certain 



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