46 Mr. Moon on Fresnel's Theory of Diffraction, 



fallacies which have been worked up into the received theory, 

 but chiefly with the intention of giving a clear view of the en- 

 tire subject; and whilst I disclaim the idea of endeavouring to 

 build up a settled theory of my own in the place of that which 

 I assume myself to have destroyed, I propose to suggest 

 various considerations which from time to time have occurred 

 to me, and which 1 have some hope may lead, in the hands 

 of those who have more time and opportunity for such re- 

 searches than are likely to fall to my lot, to the ultimate and 

 complete elucidation of the subject. 



Recurring to my former example, suppose a series of con- 

 centric, spherical, diverging waves to be diffracted by an opake 

 body indefinitely extended in all directions but one. If we 

 suppose that no reflexion takes place near the edge of the dif- 

 fracting body, and that no vibration is communicated through 

 it to the aether beyond, or, admitting the possibility of either 

 or both of these circumstances occurring, if we suppose it to 

 be ascertained, by experiment or otherwise, that the phaeno- 

 mena are in no degree attributable thereto, it is evident that 

 each wave of the series must prolong \\.%^^ continuously within 

 the shadow. On a former occasion (see vol. xxiv. p. 81 of 

 this Journal) I endeavoured to point out how, upon a parti- 

 cular assumption as to the nature of the incident waves, this 

 would naturally occur upon simple mechanical principles. 

 But I conceive the same would be true, whatever the nature 

 of the incident wave might be. For experiment assures us, 

 that light actually penetrates within the geometrical shadow 

 of the diffracting body, and upon the assumptions we have 

 above made, this can only occur through an innate power in 

 diffracted waves of diffusing themselves laterally; and this 

 being the case, it seems impossible that such diffusion should 

 operate itself otherwise than continuously. If we were to lay 

 aside the consideration of any disturbing effect which the 

 forces residing in the particles of the diffracting body might 

 produce in the diffracted waves, perhaps this position might 

 be stated absolutely ; as would be the case if Fresnel's con- 

 clusion, that the diffraction is independent of the nature of the 

 diffracting body and the form of the edge were to be relied 

 on ; and at any rate it is very difficult to conceive how these 

 forces should tend to produce discontinuity. 



Assuming then that each extended wave is continuous up 

 to the point where it finally terminates, it is evident that no 

 interference can occur, except through the intersection of 

 waves nearly imrallel to each other, which would imply, that 

 after diffraction the waves vary from the spherical form ; and 

 that this change of form is effected differently in different 



