324 Memoir of Dr. Thomas Young. 



point in space. In this point a leaf of fine paper was placed. 

 Each ray taken separately made it shine with much effect, 

 but when the two rays were re-united, when they arrived 

 simultaneously on the leaf, then light disappeared, — the 

 most complete night succeeded the day. 



Two rays are not always annihilated at their point of in- 

 tersection. Sometimes a partial weakening only is observed. 

 Every thing depends on the length of their course, and 

 this by very simple laws, the discovery of which, at any 

 period, would have been sufficient to immortalize a phi- 

 losopher. 



The differences in the direction which produce among 

 rays a confluence accompanied by their entire destruction, 

 have not the same value for light of different colours. 

 When two white rays cross, it is then possible that one of 

 their principal constituents, the red for example, exists 

 alone in the condition of destruction ; but white less. The 

 red constitutes green. Thus the interference of light is 

 manifested by the phenomena of colour, and thus, the 

 different elementary colours are exhibited without the 

 assistance of a prism to separate them. It cannot fail to 

 be remarked, that there does not exist a single point in 

 space where a thousand rays, proceeding from different 

 origins, do not cross each other after more or less oblique 

 reflexions, and one can perceive at a glance, the whole 

 extent of the unexplored region which these interferences 

 open to the investigation of philosophers. 



When Young published this theory, many phenomena of 

 periodical colours were already offered to observers ; it 

 should be added, that they had resisted all explanation. 

 In the number we may reckon the rings formed by reflexion, 

 not from thin pellicles, but from plates of thick glass 

 slightly curved ; the iridescent bands of different sizes, by 

 which the shadows of bodies are bounded exteriorly, and 

 sometimes covered interiorly, which Grimaldi observed 

 first, which subsequently attracted the genius of Newton 

 without success, and the complete theory of which was 

 reserved for Fresnei ; the red and green arcs which are 

 observed in a greater or less number, immediately, below 

 the seven prismatic colours of the chief rainbow, and which 

 appear so completely inexplicable as to have been passed 



