DIFFRACTION. 63 



ternal hole ; and, considering each of these as a new centre of 

 disturbance, to find their total resultant at any point of the screen 

 on which the fringes are received. The method of solution has 

 been pointed out by Professor Airy ; and he has shown that when 

 the external hole is a rectangular parallelogram, and the diffract- 

 ing aperture of the same form, and similarly placed, the law of 

 illumination at any point of a screen will be similar to that pro- 

 duced by a rhomboidal aperture, in Fresnel'a method of obser- 

 vation; the dimensions and distances in the two cases being 

 connected by certain relations.* From these investigations Pro- 

 fessor Airy concludes that the size of the external hole could 

 not account for the dark central shadow mentioned by Newton 

 in the sixth observation. He has confirmed this conclusion by 

 experiment ; and employing holes of various magnitudes, he found 

 the central band in all cases bright. The effect recorded by New- 

 ton is ascribed by Professor Airy to the influence of contrast on 

 the retina. 



A remarkable class of phenomena arise when a lens is placed 

 close to an aperture of any form, and the light received on a screen 

 at its focus, or on an eye-glass at its own focal distance from 

 it. In fact, the phenomena of diffraction are in this manner pro- 

 duced with holes of considerable dimensions, and were observed by 

 Sir W. Herschel, with the undiminished apertures of his great 

 telescopes ; the stars being seen encompassed by several dark and 

 bright rings, succeeding one another at equal intervals, when a 

 high magnifying power was employed. But the phenomena be- 

 come more distinct when the aperture is limited by a diaphragm 

 of moderate size, the diameters of the rings varying inversely as 

 those of the apertures. The effects produced by diaphragms of dif- 

 ferent sizes and forms have been examined in much detail by Sir 

 John Herschel and M. Arago.f 



The phenomena produced by minute apertures, when combined 

 with a lens in the manner now spoken of, have been studied with 

 much zeal and success by Fraunhofer. The most remarkable of 

 these phenomena are those produced by a fine grating, such as may 

 be formed by stretching a fine wire between two parallel screws of 



* " On the Calculation of Newton's Experiments on Diffraction." Camli<l<j<- 

 Trans., vol. v. part 2. 



t Professor Amici has also noticed some phenomena of the same class. See EdiM. 

 Journal of Science, vol. iv. p. 306. 



