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

 by the Grooved Surfaces of Glass and Steel* Part I. By Sir David 

 Brewster, K.H., E.R.S. Lond. and Edin. (Plate XXII.) 



(Read 7th March 1864.) 



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 XXII. 

 figs. 1 and 2. The glass plate ABCD, fig. 2, was covered with grooves, but the 

 steel plate below it was grooved only on the shaded portion abed, the parts AaCc, 

 'BbdJ) being polished so as to reflect to the eye at E (fig. 1), the grooves on the 

 glass when illuminated by rays, Rr, proceeding from the first pair of the para- 

 genic spectra produced by the grooves. 



When the direction of the grooves ac is nearly parallel to the plane of re- 

 flexion, 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 AacC, ~BbdD, 

 where the steel was only polished. 



When the grooves were slightly inclined to the plane of reflexion, large 

 serrated bands appeared upon the spaces AacC, RbdD, and when this inclination 

 was increased, these large bands became smaller and more numerous, crowding 

 towards Cc and dD. 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 reflexion, the 

 minute serrated bands upon abed become smaller and less serrated. 



* In a very interesting paper on the Spectra produced by Gratings or Grooved Surfaces, M. 

 Babinet has given them the appropriate name of Paragenic, 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. 



VOL. XXIV. PART I. 3 O 



