48 Dr Brewster on a new series of 



lourless image becomes extremely faint, and vice versd. The 

 general phenomena of the prismatic images, such as their dis- 

 tance from the common image, and the dispersion of their co- 

 lours, depend entirely on the magnitude of w + w, or the 

 number of grooves and intervals that occupy any given space; 

 and the laws of these phenomena have been accurately deter- 

 mined 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 obvi- 

 ous that it was not owing to any accidental cause. In the spe- 

 cimen 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 de- 

 falcation 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 S0° or 40°. I thus saw a series of very interesting pheno- 

 mena. The ordinary image of the aperture, as formed by th6 

 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 remarkable 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 

 m -\- n =z lOOOdth of an inch, no less than four complete 

 orders of colours were developed, as shown in the following 

 Table-— 



