24 Sir David Brewster on the Bands formed by 



They are seen both at a perpendicular incidence and when the 

 plates are inclined in a plane parallel to the grooves. 



These bands become narrow as the distance of the two grooved 

 surfaces is increased, and they are seen at all angles of incidence, 

 and in all planes of reflexion from the grooved surfaces. 



I have observed these bands, which are generally more or less 

 serrated, in combinations of 1000 with 1000, 1000 with 2000, 

 1000 with 500, 2000 with 500, and in the combination of four 

 surfaces of 2000, 1000, 100, and 500. 



In the combination of 1000 and 500, and in no other, a very 

 peculiar system of bands is seen with a lens. They are not ser- 

 rated, and not perpendicular to the grooves. The system con- 

 sists of two sets equally inclined to the direction of the grooves, 

 when the grooves in one plate are slightly inclined to those in 

 the other. By diminishing the inclination of the grooves, the 

 inclination of the bands to the direction of the grooves dimi- 

 nishes, and when the grooves become parallel, the bands become 

 parallel and disappear. 



These bands must have a different origin from those pre- 

 viously described, as they are similar in number upon all the 

 prismatic images. 



In these experiments the duplication of the bands on the 

 second spectrum, and their increase in arithmetical progression 

 on the other spectra, is a remarkable fact which it is difficult to 

 explain. The second spectrum differs from the first, and the 

 third from the second, only in their length ; and we can hardly 

 suppose that they have a property in a direction perpendicular 

 to their length, or to Fraunhofer's lines, which would increase 

 the number of their bands. 



The bands which we have described are more distinct when 

 the spectra are pure or formed from a narrow line or bar of light; 

 but when we wish to see the bands on the bar of light or the 

 central image A (fig. 4), the spectra must be formed from wide 

 spaces, which gave impure spectra. 



In order to examine the interference bands under different 

 conditions, I placed (as in fig. 6) a plate of polished steel at dif- 

 ferent distances from another plate of steel containing six sys- 

 tems of grooves executed by Sir John Barton, varying from 3125 

 divisions in an inch to 10,000. When the light was reflected 

 twice from the grooved surface and once from the plain steel 

 surface, the bands which covered the colourless image and the 

 paragenic spectra were splendid beyond description, and unlike 

 anything of the kind that I had previously seen. 



1. The bands were parallel to the grooves, or to the lines in 

 the spectra. 



