366 M, G. Quincke on Diffraction. 



called maxima of the second class, known to Fraunhofer*, are or- 

 dinarily made use of. The greater the wave-length, and the 

 less the distance between two adjacent groups of apertures of 

 the grating, the greater is the distance of these maxima from 

 one another. 



Besides these maxima, however, as we learn from experiment, 

 other maxima, less luminous, make their appearance, which I 

 have named secondary, and which the theory does not enable us 

 to foresee. If m denotes a whole number, the secondary maxima 



1 2 



are situated at -* — , &c. of the distance between two neigh- 



m m ° 



bouring maxima of the second class, or at the places where a 

 grating with 2, 3, ... m times the distance between the aper- 

 tures or furrows would show maxima of the second class. Their 

 situation relative to the maxima of the second class is, with the 

 same grating, the same in transmitted or reflected light for dif- 

 fraction in the most diverse substances. The incident rays may 

 make any angle we please with the normal to the surface of the 

 grating. Under otherwise like circumstances, however, the 

 value of m may change with the colour. 



The gratings were selected as various as possible ; the distance 

 between two adjacent groups of apertures varied between 0*2 and 

 O - 0O25 millim. The experiments were made upon gratings with 

 opaque bars in air or water, with apertures in an opaque layer of 

 soot, silver-collodion, silver, gold-leaf, or in iodide of silver on a 

 glass plate, and furrow- or ridge-gratings cut in glass or metal. 



I have now studied the diffraction of polarized light by these 

 gratings. 



If we look at a sodium-flame through a doubly refracting 

 prism and a grating with vertical apertures or furrows, we see 

 two series of flame-images, one over the other, polarized parallel 

 and perpendicular to the principal diffraction-plane. Two 

 flame-images, one above the other, corresponding to the same 

 maximum of the second class, usually appear equally bright. 

 Only in isolated spots, mostly with feebler intensity of light, do 

 any differences appear. If we advance to flame-images of a 

 higher order, sometimes the light polarized parallel, and some- 

 times that polarized perpendicular to the principal diffraction- 

 plane may predominate. 



Similar differences are observed in reflected light; and, 

 indeed, here again furrow- and elevation-gratings of symme- 

 trical form exhibit the same phenomena as soon as right and 

 left are exchanged. 



Slight differences in the shape of the apertures, or furrows, or 



* Gilbert's Ann. vol. lxxiv. p. 340 (1823). 



