COLOURS OF THICK PLATES. [171] 



Now while the light is still in the glass conceive the particles of dust removed, and 

 then replaced at random. The chances are that no particle will now occupy the position 

 formerly occupied by P. Let P be the particle nearest to the former position of P; and, 

 to make a supposition as favourable as possible to interference, let P be the very particle 

 P moved a little along the surface without rotation. Although the interval of retardation 

 R of the two streams diffracted by P in its first position, and reaching a given point of 

 space, is sensibly the same as the interval of retardation of the two streams diffracted by 

 P in its second position, and reaching the same point, yet this interval would be changed 

 altogether were the transference of position to take place during the interval of time which 

 elapses between the departure of the wave from P and its return after reflexion, as may 

 very readily be seen. The amount, too, by which the interval of retardation would be 

 changed would vary in an irregular manner from one particle to another, and therefore 

 no regular interference would take place. Now the purely ideal case just considered is pre- 

 cisely analogous to the case of actual experiment when a luminous point is viewed through 

 a plate of glass with both faces dimmed, since the particles on one face have no relation to 

 those on the other. We ought not therefore in such a case as this to expect to see rings or 

 bands. 



29. According to the formula (35), the angular breadth of one of the bands formed by 

 a plane mirror becomes considerable when i becomes nearly equal to 90°, so that, apparently, 

 bands ought to be visible at a large angle of incidence. But if the courses of the two streams 

 scattered by the same set of particles be traced, it will be found that they are so widely 

 separated that, for various reasons, no regular interference can be expected to take place. 

 Accordingly, the bands are not seen at a large angle of incidence. 



30. In the preceding sections I have spoken of the light by which the rings are formed as 

 having been scattered at the dimmed surface. And so it really is, if by that term we 

 merely understand deflected from the course it would have followed according to the regular 

 law of refraction. But according to the explanation given in the preceding article the light 

 is not scattered, in the strict sense of the term, but regularly diffracted. Scattered light is, 

 strictly speaking, such as that by which objects are commonly seen, or again, such as 

 that which is transmitted through white paper and similar substances. The preceding view 

 of the nature of the light by which the rings are formed is confirmed by the results of several 

 experiments. 



In the experiments of Sir William Herschel and M. Pouillet mentioned in the introduc- 

 tion, as well as in some of those of the Duke de Chaulnes, rings of the same nature as 

 those formed by a tarnished mirror of quicksilvered glass were produced in cases in which 

 the deflection of the light from its regular course was incontestibly of the nature of diffraction. 

 From the similarity of effect we have a right to infer a probable similarity of cause, unless 

 such a supposition should entail some peculiar difficulty, which does not seem to be the case 

 in the present instance, but quite the contrary. 



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