M. G. Quincke 0)1 Diffraction. 365 



of maximum and minimum intensity so resulting will be abso- 

 lutely identical. 



6 New Square, Lincoln's Inn, 

 March 15, 18/3. 



P.S. — Presnel, in his "Memoire sur la Diffraction de la 

 Lumiere" [Memoires de V Academic, Paris, 1826, p. 406), dis- 

 tinguishes between "Pintensite des vibrations" as depending on 

 the simple power, and "Pintensite de la lumiere" as depending 

 on the square of the particle-velocity. Can any intelligible 

 ground be assigned for this distinction ? 



XLVI. On Diffraction. By G. Quincke/*. 



IN a searching investigation of the phenomena which occur 

 in the inflection (diffraction) of light, I have arrived at re- 

 sults which deviate from the representations hitherto given, in 

 several, and, I think, important points. 



Some time since (see Pogg. Ann. vol. cxlvi. pp. 1-65, 1872), 

 I treated theoretically the phenomena which are perceived when 

 a point or a line of light is looked at, with a telescope or with 

 the naked eye, through a diffraction-grating — that is, a combi- 

 nation of apertures of the same size and shape and at equal dis- 

 tances from each other. The theory therefore comprises gratings 

 with opaque or transparent bars, as well as such as are cut with 

 a diamond-point in a plane glass or metal plate. Besides the 

 validity of Huyghens's principle, it was therein presupposed 

 that a furrow-grating consists of depressions with little stair- 

 like steps, one face of which is parallel to the untouched face of 

 the plate. 



Por furrow-gratings the formulae are much more complicated 

 than for gratings with opaque bars. They show, in accordance 

 with the experiments, that the luminous intensity with these 

 gratings (which in practice are used in preference) depends very 

 considerably on the dimensions of the furrows and on the sub- 

 stance with which they are filled up, whether the light be trans- 

 mitted through the grating or reflected from it. 



The investigation of reflected light affords this advantage, that 

 the experiment can be better accommodated to the presuppositions 

 of the calculation than with transmitted light. Symmetrically 

 formed furrows, and elevation-gratings of the same material 

 (which can be very completely produced galvanoplastically with 

 the aid of some experimental artifices), exhibit the same proper- 

 ties when right and left are exchanged. 



Por the determination of the wave-lengths of light the so- 

 * Translated from a separate impression communicated by the Author. 



