228 Biographical Memoir of Dr Thomas Young, 



tion, differ in amount for lights differently coloured. When 

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

 their principal component parts, the red ray for example, may 

 alone be in those circumstances in which it will be destroyed. 

 But white deprived of red becomes green ! This luminous in- 

 terference, then, manifests itself by the phenomena of coloration; 

 and thus the different elementary colours exhibit themselves, 

 without a prism having separated them. And now then, let it 

 be well considered, that there does not exist a single point of 

 space where a thousand rays of the same origin do not cross af- 

 ter being reflected more or less obliquely ; and then we may at 

 once perceive all the extent of that unexplained region which 

 the doctrine of interferences opens up to the investigations of 

 philosophers. 



At the time when Dr Young published this theory, many of 

 the phenomena of recurrent colours had already presented them- 

 selves to the attention of observers; and, we must add, had re- 

 mained wholly unexplained. Among the number we may men- 

 tion those which are formed by reflection, not more on thin pel- 

 licles than on thick mirrors of glass which are somewhat curved; 

 also the iridescent bands of different sizes with which the sha- 

 dows of bodies are fringed externally, and sometimes covered 

 internally, which Grimaldi first perceived, and which also exer- 

 cised the genius of Newton, but the satisfactory theory of which 

 was proposed by Fresnel ; also the red and green coloured 

 arches, more or less numerous, which are seen immediately 

 underneath the seven prismatic colours composing the principal 

 rainbow in the sky, and which had appeared so completely in- 

 explicable, that philosophers had ceased to make any mention 

 of them in the common treatises on physics; and finally, those 

 coronas of marked colours, with diameters perpetually varying, 

 which often appear round the sun and the moon. And when 

 I remember that there are those who do not value scientific 

 theories, except so far as they have an immediate practical ap- 

 plication, I must not terminate this enumeration of the pheno- 

 mena which characterize the more or less numerous series of pe- 

 riodic colours, without mentioning those rings which are so re- 

 markable for the regularity of their form, and the purity of their 



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