100 Sir David Brewster on the Bands funned by 



rous, and more coloured, till they are finally parallel to A M, the 

 fringes being twice as numerous on the second spectrum as on 

 the first. 



When the grooves are perpendicular to A M, as in fig. 7, the 

 bands are faint and indistinct. The light being incident per- 

 pendicularly and the gratings turned round AM on a plane 

 perpendicular to A M, the fringes do not increase in number or 

 greatly change, if the motion is accurately in a plane perpendi- 

 cular to AM. 



When the gratings are turned in the plane of the horizon 

 passing through A M, the side N M approaching the eye, the 

 fringes on the left-hand spectra descend, increasing rapidly in 

 number; and when the side MN recedes from the eye, the 

 fringes ascend, increasing in magnitude and diminishing in 

 number, and are highly coloured. At a certain angle they 

 become parallel to the grooves, when by continuing the rota- 

 tion they move downwards, increasing in number, and again 

 becoming parallel to the grooves. 



In the preceding experiments the bands are seen on the sur- 

 face of the gratings; but when the grooved surfaces are in con- 

 tact and the grooves parallel, bands of an entirely different kind 

 are seen, not on the surface of the gratings, but by rays diver- 

 ging from the luminous disk. If we use a long and narrow bar 

 of light, such as the opening between the window-shutters, then, 

 when the grooves are parallel to the bar, and the grooved sur- 

 faces perpendicular to the plane of incidence, the bands are 

 parallel to the bar and its spectra. By inclining the grooves to 

 the luminous bar, the bands are inclined to the spectra, dividing 

 each of them into a great number of spectra; and at an azimuth 

 of 45° the bands become perpendicular to the spectra. At all 

 these inclinations the bands on the second spectrum are double 

 those on the first, the number increasing in arithmetical pro- 

 gression on succeeding spectra. 



When the angle of incidence is increased, the bands increase 

 in number^ but very slightly with gratings of 1250 divisions in 

 an inch. 



By increasing the distance between the gratings, the bands 

 also increase in number. 



Bands similar to those now described are produced with inter- 

 esting phenomena by a single grating placed (as in fig. 8) so 

 that the image of the grooved surface A B, reflected from M N 

 the lower surface of the glass, is superposed as it were upon the 

 grooved surface itself. 



1. When the plane of reflexion is perpendicular to the grooved 

 surface, and the grooves in the same plane, the bands on the 

 spectra are parallel to the bar of light A B, those on the second 



