60 REPORT ON PHYSICAL OPTICS. 



same as that reflected by a plate of air, whose thickness bears a 

 certain simple relation to the radius of the aperture, and to its 

 distances from the luminous origin and from the eye. With 

 homogeneous light, therefore, the illumination of the central spot 

 vanishes periodically, as the distance of the eye from the aperture 

 is varied ; and in white light it assumes in succession the most 

 vivid and beautiful hues, coinciding with those of the reflected 

 rings of thin plates. These interesting phenomena were observed 

 about the same time by Sir John Herschel, and their laws deduced, 

 independently, from observation.* 



With the exception of the observations now referred to, no 

 attempt has been made to verify the theory, by comparing the 

 intensity of the light in the fringes with that deduced from the 

 formulae ; and indeed it is obvious that a comparison of this nature 

 is ill calculated to afford any conclusive evidence on the question. 

 Fresnel thought, however, that the expression for the intensity 

 might be indirectly verified, by superposing two sets of fringes 

 (such as the interior and exterior fringes of a narrow opaque body), 

 by means of double refraction, and then examining the position of 

 the new maxima and minima. This ingenious suggestion does not 

 appear to have been acted on. 



The intensity of the light in the partial waves sent from each 

 point of the primary wave, considered as a distinct centre of 

 disturbance, will necessarily be different in different directions, 

 depending on the angle which these directions form with the front 

 of the original wave ; and to solve the problem of diffraction in 

 its most general form, it would be necessary to know the law of 

 this variation. Fresnel has shown, however, that the rays whose 

 directions are inclined at sensible angles to the normal to the 

 front of the primary wave, destroy one another by interference ; 

 so that the actual effect is produced by rays indefinitely near that 

 normal, and which therefore may be regarded as of equal inten- 

 sity. The truth of this assumption, however, is disputed by M. 

 Poisson. From his theory of the propagation of motion in fluid 

 media, this mathematician inferred that the absolute velocities of 

 the molecules are insensible in directions making finite angles 

 with the direction of the original vibrations. He concludes, 

 therefore, that these velocities, or the intensity of the light in the 



* Essay on Light, Art. 729. 



