52 Dr Brewster on a new series of 



A similar obliteration of tints takes place on the ordinary 

 image A B. 



The 1st obliteration, vizi that of the violet, takes place at o. 

 Fig. 9, and that of the red at p ; while the intermediate co- 

 lours disappear at intermediate points. This first space of ob- 

 literation has no corresponding one at the same incidence in 

 any of the prismatic images. 



The 2d obliteration of the violet in A B takes place at q, 

 and that of the red at r, and this corresponds in incidence with 

 the obliterations m' n'^ m' n' on the second prismatic image. 



The 3d obliteration of the violet takes place at .?, and that 

 of the red at ^, and this corresponds in incidence with the four 

 obliterations on the second and fourth prismatic images, viz. 

 /a 1/ At.' / m!" n"\ m'" n"'. 



In all these phenomena the points tw, w, /a, v, &c. are only 

 the points of minimum intensity, or of maximum obliteration ; 

 for the tints never entirely disappear, and those obliterated at 

 each line m n form an oblique spectrum containing all the 

 prismatic colours. 



The analysis of these curious and apparently complicated 

 phenomena becomes very simple when they are examined under 

 homogeneous illumination. The effect produced in red light 

 is represented in Fig. 1 2, where A B is the image of the rec- 

 tangular 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, consist of ho- 

 mogeneous red light, which is obliterated at the fifteen shaded 

 rectangles, which are the minima of the new series of periodi- 

 cal colours which cross both the ordinary and the prismatic 

 images. The centres /?, r, ^, w, v, &c. of these rectangles cor- 

 respond with the points marked with the same letters in Fig. 

 9, and if we had drawn the same figure for violet hght, the 

 centres of the rectangles would have corresponded with o, §', 5, 

 ?w, ^, &c. in Fig. 9. 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. 



I^ it should be practicable to remove a still greater portion 

 of the faces w, the first minimum ^, Fig. 12, would commence 



