GROOVED SURFACES OF METALLIC AND TRANSPARENT BODIES. 307 



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. Fig. a. 



The analysis of these curious and apparently com- A 



plicated phenomena becomes very simple when they 

 are examined under homogeneous illumination. The 

 effect produced in red light is represented in Fig. 2, 

 where A B is the image of the rectangular aperture | 



reflected from the faces n of the steel, and the four 

 images on each side of it correspond with the pris- 

 matic images. All these nine images, however, con- 

 sist of homogeneous red light, which is obliterated 

 at the fifteen shaded rectangles, which are the mi- 

 nima of the new series of periodical colours which 

 cross both the ordinary and the prismatic images. 

 Tlie centres p, r, t, n, v, &c. of these rectangles cor- 

 respond with the points marked with the same letters 

 in Fig. 1; and if we had drawn the same figure for 

 violet light, the centres of the rectangles would have 

 corresponded with o, q, s, m, (Jj, &c. in Fig. 1. The 

 rectangles should have been shaded off to represent 

 the phenomena accurately, but the only object of the 

 figure is to show to the eye the position and relations of the minima of the periods. 



If it should be practicable to remove a still greater portion of the faces n, 

 the first minimum p, Fig. 2, would commence at a greater angle of incidence ; 

 and other two rows of minima, namely, rows of five and six, would be found 

 extending to the fifth and sixth prismatic images. The arrangement and suc- 

 cession of these is easily deducible from Fig, 2, where the law of the pheno- 

 menon is obvious to the eye. 



The following table contains the angles of incidence reckoned from the per- 

 pendicular at which these minima occur in the extreme rays. 



Position of the minima in red light. 



Ord. Ira. 



First minima p . . . . 76 

 Second minima r . . 55 45 

 Third minima 23 30 



2 r2 



