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II. On the Bands formed by the Superposition of Par a genie Spectra 

 produced by the Grooved Surfaces of Glass and Steel*. — Part I. 

 By Sir David Brewster, K.H., F.R.S.L. # E.-f 

 [With a Plate.] 



IN examining the colours produced by thin laminae of the 

 crystalline lens of fishes, I observed a series of rectilineal 

 serrated fringes perpendicular to the direction of the fibres, and 

 produced by inclining the laminae in a plane cutting these fibres 

 at right angles. I was thus led to imitate these fringes or bands 

 by combining grooves or striae cut upon glass or steel surfaces, 

 or grooves taken from these surfaces upon isinglass or gums. 



In my first experiments I combined a system of grooves on 

 glass, executed for me by Mr. Dollond, with a similar system 

 on steel executed by Sir John Barton, both of them containing 

 2000 divisions in an inch. The plate of glass was placed above 

 the plate of steel, and slightly inclined to it, as shown in Plate I. 

 figs. 1 and 2. The glass plate A B C D, fig. 2, was covered with 

 grooves, but the steel plate below it was grooved only on the 

 shaded portion abed, the parts A a c C, B b dJ) being polished 

 so as to reflect to the eye at E (fig. 1), the grooves on the glass 

 when illuminated by rays, B r, proceeding from the first pair of 

 the paragenic spectra produced by the grooves. 



When the direction of the grooves a c is nearly parallel to the 

 plane of reflexion, and to one another, a series of minute serrated 

 bands is seen on the space abed, where the light has been 

 transmitted twice through the grooves on glass, and reflected 

 once from those on steel ; but no bands are seen upon A a c C, 

 B b dD, where the steel was only polished. 



When the grooves were slightly inclined to the plane of re- 

 flexion, large serrated bands appeared upon the spaces A a c C, 

 ~BbdD-, and when this inclination was increased, these large 

 bands became smaller and more numerous, crowding towards 

 C c and d D. On the other hand, they become larger and 

 larger as the direction of the grooves returned into the plane of 

 reflexion. In the azimuth of 0° they become straight, and by 

 increasing the azimuth, they pass, as it were, to the right hand, 

 as shown in fig. 3. 



When the direction of the grooves is inclined to the plane of 



* In a very interesting paper on the Spectra produced by Gratings or 

 Grooved Surfaces, M. Babinet has given them the appropriate name of pa- 

 ragenic, in order to distinguish the spectra produced by refraction from 

 those produced by the lateral propagation of light. " Sur la Paragenie ou 

 propagation laterale de la lumiere." Paris, 1864. Extrait du Cosmos. 



f From the Transactions of the Royal Society of Edinburgh, vol. xxiv. 

 part 1 . Communicated by the Author, 



