Biographical Memoir ofDr Thomas Young. 227 



reflected by the second surface, encountering the rays reflected 

 by the first, were completely annihilated. It is this conflict of 

 the rays which the author designated by the name, now so fa- 

 mous, of interference. 



This, then, beyond contradiction is a most strange hypothesis. 

 Who would not be surprised to find darkness in the sun's rays, 

 — in points which the rays of the luminary freely reach ; and 

 who would imagine that any one could suppose that the dark- 

 ness could be produced by light being added to light ! 



A philosopher may well be proud when he can announce a 

 result, which to such an extent as this surprises our common 

 apprehensions ; but he ought, without delay, to support it with 

 demonstrative proofs, under the risk of being compared to those 

 Eastern writers, whose wild reveries so much delighted for a 

 thousand and one nights the sultan Schahriar. 



But Young had not this prudence ; and at first he only de- 

 monstrated that his theory would explain the phenomena, and 

 proceeded no farther than possibilities. So that when, at a later 

 period, he proceeded to his positive proofs, the public had 

 adopted prejudices which he could no longer overcome. It is 

 nevertheless true that the experiment upon which our learned 

 colleague at that time founded his memorable discovery, places 

 it beyond the shadow of a doubt. 



Two rays coming from the same source may proceed by 

 routes somewhat different, and may cross at a certain point in 

 space. If in this point we place a sheet of fine paper, each ray, 

 taken alone, will illuminate it brightly ; but when the two rays 

 unite — when they arrive simultaneously upon the paper — all il- 

 lumination disappears, and the most complete darkness takes the 

 place of light. 



Two rays do not, however, always completely destroy each 

 other in the point of intersection. Sometimes there is only a 

 partial obscuration ; and sometimes the one ray is added to the 

 other. Every thing depends on the difference of the lengths of 

 the course which they have run ; and that according to very 

 simple laws, the discovery of which would at any time have been 

 sufficient to have immortalized any philosopher. 



The differences in the course which produces among rays 

 those conflicts which are accompanied with their entire destruc- 



