34 PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 



Investigation of the Absolute Intensity of Interfering Light, printed in the fifteenth Volume 

 of the Transactions of the Royal Society of Edinburgh, p. 315. In this memoir the author 

 investigates the case of a series of plane waves which passes through a parallelogram in front 

 of a lens, and is received on a screen at the focus of the lens, as well as several other particular 

 cases. By equating the total illumination on the screen to the area of the aperture multiplied 

 by the illumination of the incident light, the author arrives in all cases at the conclusion that 

 in the coefficient of vibration of a secondary wave the elementary area dS must be divided 

 by \r. In consequence of the employment of intensities, not displacements, the necessity for 

 the acceleration of the phase by a quarter of an undulation does not appear from this 

 investigation. 



In the investigations of Mr. Smith and Professor Kelland, as well as in the verification 

 of the formula (46) given in the last article, we are only concerned with that part of a 

 secondary wave which lies near the normal to the primary. The correctness of this formula 

 for all directions must rest on the dynamical theory. 



36. In any given case of diffraction, the intensity of the illumination at a given point 

 will depend mainly on the mode of interference of the secondary waves. If however the 

 incident light be polarized, and the plane of polarization be altered, every thing else remaining 

 the same, the mode of interference will not be changed, and the coefficient of vibration will 

 vary as sin tp, so that the intensity will vary between limits which are as 1 to cos 2 6. If 

 common light of the same intensity be used, the intensity of the diffracted light at the given 

 point will be proportional to | (l + cos 2 6). 



