GROOVED SURFACES OF METALLIC AND TRANSPARENT BODIES. 303 
m + n = lOOOdth of an inch, no less than four complete orders of colours 
were developed as shown in the following Table. 
White 
90 do 
Bluish green 
o 
. . . 54 
30 
Yellow 
801 
Yellowish green . . . 
. . . 53 
1 5 
Reddish orange 
77i 
Whitish green .... 
. . . 51 
Pink 
76 20 
Whitish yellow . . . 
. . . 49 
Junction of pink and blue 
75 40 
Yellow 
. . . 47 
15 
Brilliant blue 
74 30 
Pinkish yellow .... 
. . . 41 
Whitish 
71 
Pink red 
. . 36 
Yellow 
64 45 
Whitish pink .... 
. . . 31 
Pink 
59 45 
Green 
. . . 24 
Junction of pink and blue 
58 10 
Yellow 
. . 10 
Blue 
56 
Reddish 
. . . 0 
These colours are obviously those of the reflected rings in thin plates. By 
turning the steel plate round in azimuth, the very same colours are seen at the 
same angles of incidence, and they suffer no change either by varying the 
distance of the luminous aperture, or the distance of the eye of the observer. 
I now examined various other specimens which possessed the same property. 
In some there were three orders of colours, in others two, and others one, 
while in some only one or two tints of the first order were developed. These 
different effects are more minutely detailed in the following Table. 
Number of grooves Orders and portions of orders of colours developed from 
in an inch. 90° up to 0° of incidence. 
500 Citron yellow of the first order. 
625 One complete order, and up to reddish yellow of the second order. 
Colours very dilute. 
1000 Four complete orders of colours. 
1 000 One complete order, together with blue green and yellowish green of 
the second order. 
1250 One complete order, together with blue and bluish green of the second 
order. Colours exceedingly faint and diluted. 
2000 One complete order, together with blue green and greenish yellow of 
the second order. 
