1886.] on Thomas Young. 5C7 



himself as to the existence of shadows in a fluid medium. He 

 held therefore that light was due to the darting forth of minute 

 particles in straight lines; and he threw out the idea that colour 

 might be due to the difference of bigness in the particles. He endowed 

 these particles with what he called Jits of easy transmission and 

 reflection. The dark rings, in his immortal experiment, were produced 

 where the light-particles were in their transmissive " fit." They went 

 through both surfaces of the film of air, and were not thrown back to 

 the eye. The bright rings occurred where the light-particles were 

 in their reflective fit, and where, on reaching the second surface of 

 the film, they were thrown back to the eye. The cardinal point 

 here is, that Newton regarded the recurrence of light and darkness 

 as due to an action confined to the second surface of the film. And 

 here it was that Young came into irreconcilable collision with him, 

 proving to demonstration that the dark rings occurred where the 

 portions of light reflected from both sides of the film extinguished 

 each other by interference, while the bright rings occurred where 

 the light reflected from the two surfaces coalesced to enhance the 

 intensity. 



Young next applied the wave theory to account for the diffraction 

 or inflection of light, that is to say, the effects produced by its bending 

 round the edges of bodies. When a cone of rays issuing from a very 

 minute point impinges on an oj^aque body, so as to embrace it wholly, 

 the shadow of the body, if received upon a screen, exhibits fringes of 

 colour. They follow so closely the contour of the opaque body, that 

 Sir John Herschel compared them to the lines along the sea-coast in 

 a map. If a very thin slip of card, or a hair, be placed within such a 

 cone, it is noticed that besides the fringes outside the shadow, bands of 

 colour occur within it ; the central, or brightest band, being always 

 white when white light is employed. It is a singular and somewhat 

 startling fact, that by the interposition of an opaque body, say a small 

 circle of tinfoil, the point on which we should expect the centre of the 

 shadow to fall, is, by the joint action of diffraction and interference, 

 illuminated in precisely the same degree as it is when the opaque 

 circle is withdrawn.* In reference to the interior fringes. Young made 

 the observation, which is of primary importance, that, if you intercept 

 the light passing by one of the edges of the strip of card or of the 

 hair, the fringes disappear. It requires the inflection of the waves 

 round both edges of the object, and their consequent interference, to 

 produce the fringes. 



Young's attempt to explain the phenomena of diffraction was a 

 distinct advance on the extremely artificial hypothesis of Newton. 

 Still his attempt was not so successful as his explanation of the 

 colours of striated surfaces and of thin, thick, and mixed plates. 

 Here the young officer of engineers to whom I have already referred, 



* A similar diffraction has been proved by Lord Kayleigh to occur in the case 

 of sound. 



