264 Mr. J. Barton on the Inflexion of Light. 



they will form by their union a single wave of double force. 

 Now light is supposed to consist of the undulations of an ima- 

 ginary elastic aether, as sound consists of undulations of the air. 

 If therefore we suppose a ray entering into a darkened 

 chamber through a small orifice at A (see the following figure) 

 to proceed forward in a direct line to Y, when it falls on a 

 sheet of white paper; while another ray proceeding in the 

 direction AK, is inflected at K by touching the edge of a 

 knife, or other solid body, and turned into the direction KY, 

 so as to fall on the same point Y as the former ray ; — then the 

 effect produced by the joint action of these two rays will be 

 different, accordingly as the lengths of their paths differ or not 

 by an integral number of undulations. If the lengths of their 

 paths differ by a half-undulation, or any odd number of half- 

 undulations, they will destroy one another, and the spot Y will 

 be dark. If the lengths of their paths differ by a whole un- 

 dulation, or any number of whole undulations, they will coin- 

 cide, and the spot Y will be of double brightness. And thus 

 are explained the alternate bands of light and shade, which 

 border the shadows of bodies placed in a small beam of light 

 entering a darkened room. 



The lengths of the two rays AY, AKY, are always com- 

 puted by Young and Fresnel from the point A, which they 

 denominate the " origin of the rays :" or the " luminous point." 

 But it appears to me that the true origin of the rays is at two 

 points R, R', on the surface of the sun ; and that instead of 

 comparing AY with AKY, we ought to compare R'Y with 

 RKY. Now that this comparison should give the same re- 

 sults as the former, — in other words, that the line RA should 

 either be equal to R'A, or that their lengths should always 

 differ precisely by an integral number of undulations, — is evi- 

 dently impossible. 



It will not be said, I presume, that the two rays RA, R'A, 

 on entering through the small opening at A, exercise any 

 mutual action on each other, so as to become in fact a single 

 ray. Such a suggestion would be at variance with the whole 

 theory ofHuyghens, which necessarily assumes, as one of its 

 fundamental principles, that any number of undulations may 

 pass through each other without disturbance: — inconsistent, 

 indeed, with well known facts, such as the perfect image of 



