Mr. Moon o« Fresnel's Theory of Diffraction. 49 



rWaves, and this even when the light is what is commonly called 

 homogeneous, or of one colour. If this view of the subject be 

 correct, we have only to suppose the wave's composing light 

 of one colour to be propagated in the same ever-recurring 

 cycle, the successive members of such cycle being so related, 

 that whilst they possess the property when undiffracted of 

 traversing transparent media with the same velocity, yet they 

 differ from one another so far in the nature of their vibration 

 (or, assuming in order to fix our ideas a specific hypothesis 

 as to the nature of the undulations, the relative condensations 

 and rarefactions of their several parts), as that after diffrac- 

 tion they change their forms in different degrees; and if we 

 further suppose that this relative change of form is so adjusted 

 as that each wave of the cycle is intersected by its immediate 

 consecutive very soon after passing the diffracting body, that 

 the loci of such intersections are nearly coincident and form 

 a hyperbolic line of sensible breadth, and that the loci of the 

 intersections of each wave, with its second, third, &c. conse- 

 cutives, form other such lines respectively, laying without the 

 former and each other, and the known phasnomena are at 

 once accounted for*. 



Of the remarkable view of the constitution of light which 

 this mode of considering the subject, if well-founded, unfolds 

 to us, it is not my present purpose to speak ; I shall rather 

 address myself to the discussion of two objections which may 

 be urged against it. 



The first is, that the lines of interference actually start im- 

 mediately from the edge of the diffracting body, whereas upon 

 the principle above explained, they can only occur at a finite 

 distance from it. To this I shall only reply, that assuming 

 the objection to be founded in fact, of which I am disposed to 

 think there is no irrefragable proof, it in reality amounts to 

 very little, for though the lines of interference according to; 

 the above theory do really commence at a finite distance from- 

 the edge of the body, it is perfectly possible, and in fact highly;; 

 probable, that such distance should be of insensible magni-' 

 tude. 



The second objection to which I propose to advert may to* 

 some appear to have more weight, and 1 freely admit that,s 

 yrimajacie^ it is deserving of some consideration. It is this : 

 the change in the form of the waves after diffraction, to which 

 we have ascribed the interference in the above example, and 

 which occurs as well in that part of each wave without the geo- 



* The reader who may be desirous of seeing this mode of explanation 

 more elaborated, will find a somewhat more detailed account of it in the 

 volume of this Journal last referred to. 



l^hil. Mag, S. 3. Vol. 27. No. 177. July 1 845. E 



