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XIX. — On the Bands formed by the Superposition of Paragenic Spectra produced 

 by the Grooved Surfaces of Glass and Steel. Part II. By Sir David Brewster, 

 K.H., F.R.S. Lond. and Edin. (Plate XXIII.) 



(Read 17th April 1865.) 



In the preceding paper I have described the bands produced by gratings or 

 grooved surfaces with 500 divisions in an inch, when the two grooved surfaces 

 are in contact, and the grooves in the one slightly inclined to those in the other. 



The following results were obtained with two gratings, one of which had 2000 

 and the other 1000 divisions in an inch. 



1. When the surfaces are in perfect contact, and the grooves parallel, very 

 irregular bands are seen on the united surfaces, either with a lens or by ordinary 

 vision, and are parallel to the grooves. They are seen only on the 2d, 4th, 6th, 

 &c, spectra on each side of the luminous bar or disc. 



By turning the nearest grating slightly to the right from the azimuth 0°, the 

 bands fall back to the left, increasing in number, and descending with their con- 

 cave sides downwards into distinct serrated black and white bands, nearly 

 perpendicular to the grooves. When the nearest grating is turned to the left, the 

 bands descend towards the right, with their concave sides upwards, till they 

 become nearly perpendicular to the grooves. In all these positions, the bands are 

 twice as numerous on the fourth spectrum as on the second, and thrice as nume- 

 rous on the sixth as on the second ; and when the grooved surfaces are perfectly 

 parallel, the bands are immoveable on the grooved surfaces at all angles of inci- 

 dence. 



2. When the grooved surfaces are separated by the thickness of one or both of 

 the plates of glass, the bands are very indistinctly seen, and they seem to dimi- 

 nish in size with the distance of the grooved surfaces ; but this is not certain, 

 owing to the difficulty of fixing the plates with the grooves at the same inclination 

 to each other. 



Similar bands were seen on the united surfaces of gratings of 2000 and 2000, 

 1000 and 1000, 500 and 500, 1000 and 500, and 2000 and 500 divisions in an inch, 

 but always less distinctly when the grooved surfaces are separated by the 

 thickness of one or both of the plates. 



The beauty and distinctness of these bands depend upon the skill with which 

 the gratings are ruled. In several of the gratings which I possess, the phenomena 

 I have described can hardly be recognised. 



VOL. XXIV. PART I. 3 Q 



