30() DR. BREWSTER ON PERIODICAL COLOURS PRODUCED BY THE 
points. In this second succession the line g v begins and ends at the same 
angle of incidence, as the line m" n" in the third prismatic image a" V ; and the 
line g' v' on the second prismatic image corresponds with ml" n'" on the fourth 
prismatic image. 
This singular obliteration of the colours is 
shown more clearly in Fig. 3, where rmvn is a 
part of one of the prismatic images, rv the red 
space, g g the green space, b b the blue, and vv 
the violet space. The line of obliteration m n in 
beginning at m obliterates the extreme violet at 
m; so that the curve of illumination a bm, Fig. 
4, is just affected at one extremity m. The line 
advances into the spectrum, and at the point 
corresponding to <7, Fig. 4, a portion of the blue 
and violet is obliterated, as shown by the notch 
in the curve ; at e a portion of the green and 
blue ; at h a portion of the red and green, and 
at n the extreme red. 
A similar obliteration of tints takes place on the ordinary image A B. 
The 1st obliteration, viz. that of the violet, takes place at o, Fig. 1, and that 
of the red at p ; while the intermediate colours disappear at intermediate points. 
This first space of obliteration has no corresponding one at the same incidence 
in any of the prismatic images. 
The 2nd 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 ml n', m! n' 
on the second prismatic image. 
The 3rd obliteration of the violet takes place at s, and that of the red at t, 
and tliis corresponds in incidence with the four obliterations on the second and 
fourth prismatic images, viz. g v , g' v’, ml" ?i'", in'" nl". 
In all these phenomena the points m, n, g , v, &c., are only the points of mi- 
nimum 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. 
