GROOVED SURFACES OF METALLIC AND TRANSPARENT BODIES. 307 
B 
v 
Fig. 2. 
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. 
The 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, &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. Im. 
1st Prism. Im. 2nd Prism. Im. 
3rd Prism. Im. 4th Prism. Im. 
First minima p . . . 
. 76 0 
o / o / 
66 0 55 45 
4°1 35 23 30 
Second minima r . 
. 55 45 
41 35 23 30 
Third minima .... 
. 23 30 
2 r 2 
