REPORT ON PIIVSICAL OPTICS. 335 



transmitted through the external 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 diffracting aperture of the same form and 

 similarly placed, the law of illumination at any point of a screen 

 will be similar to that produced by a rhomboidal aperture, in 

 Fresnel's method of observation ; the dimensions and distances 

 in the two cases being connected by certain relations*. From 

 these investigations Professor 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 con- 

 firmed this conclusion by experiment ; and employing holes of 

 various magnitudes, he found the central band in all cases bright. 

 The effect recorded by Newton 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 eyeglass at its own focal distance 

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

 produced with holes of considerable dimensions, and were ob^ 

 served by Sir W. Herschel, with the undiminished apertures of 

 his great telescopes ; the stars being seen encompassed by seve- 

 ral dark and bright rings, succeeding one another at equal in- 

 tervals, when a high magnifying power was employed. But the 

 phenomena become 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 pro- 

 duced by diaphragms of different sizes and forms have been 

 examined in much detail by Sir John Herschel and M. Aragof- 



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 equal thread. When such a grating is placed before 

 the object-glass of a telescope, and a narrow slit whose length 

 is parallel to the wires of the grating, viewed through it, the 

 direct image of the slit is bordered on either side by a succes- 



* " On the Calculation of Newton's Experiments on Diffraction," Cambridge 

 Trans., vol. v. part 2. 



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

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



