302 DR. BREWSTER OX PERIODICAL COLOURS PRODUCED BY THE 
of the original surface that is left. If the image of a candle is seen by reflexion 
from such a surface, the trace of the plane of reflexion being parallel to the 
grooves, we observe the colourless image of a candle in the middle of a row 
of prismatic images arranged in a line perpendicular to the grooves. The 
colourless image of the candle is formed by the original portions n of the 
metallic surface, while the prismatic images are formed by the sides of the 
grooves m. This may be demonstrated ocularly by increasing m, and con- 
sequently diminishing n till the latter nearly disappears. In this case the 
intensity of the prismatic images rises to a maximum, while the ordinary 
colourless image becomes extremely faint, and vice versa. The general 
phenomena of the prismatic images, such as their distance from the common 
image, and the dispersion of their colours, depend entirely on the magnitude 
of m + n, or the number of grooves and intervals that occupy any given 
space ; and the laws of these phenomena have been accurately determined by 
M. Fraunhofer. 
In the course of my examination of the prismatic images, I observed in 
some specimens an unaccountable defalcation of particular colours, varying 
with the angle of incidence, and sometimes affecting one of the images and not 
the others. It sometimes appeared in close and sometimes in wide systems of 
grooves, and from the symmetry of its effects, it became obvious that it was 
not owing to any accidental cause. In the specimen in which it was most 
distinctly seen, I was surprised to observe that the white image reflected from 
the original surface of the steel was itself slightly coloured ; that its tint varied 
with the angle of incidence, and had some relation to the defalcation of colour 
in the prismatic images. 
Hitherto I had used a small disc of light, but in order to observe through 
a great range of incidence I employed a long narrow rectangular aperture, 
which gave a convergent beam of 30° or 40°. I thus saw a series of very in- 
teresting phenomena. The ordinary image of the aperture, as formed by the 
spaces n, was crossed in a direction perpendicular to its length, with broad 
coloured fringes varying in their tints from 90° to 0° of incidence. This re- 
markable effect I observed in various specimens, having from 500 to 10,000 
grooves in an inch. In a specimen with 1000 grooves in an inch, or in which 
