302 DR. BREWSTER ON 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 coloui'less image of a candle in the middle of a row 

 of prismatic images ari-anged in a line perpendicidar to the grooves. Tlie 

 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 vei-s^. 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 apertui-e, 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 



