306 DR. BREWSTER ON PERIODICAL COLOURS PRODUCED BY THE 



Fig. 3. 



id 



;h 



points. In this second succession the line ^ v begins and ends at the same 

 angle of incidence, as the line m" «" in the third prismatic image a" b' ; and the 

 line fJ v' on the second prismatic image corresponds with mi" ri" on the fourth 

 prismatic image. 



This singular obliteration of the colours is 

 shown more clearly in Fig. 3, where r m v « is a 

 part of one of the prismatic images, r v the red 

 space, gg the gi'een space, bh 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 abm, Fig. 

 4, is just affected at one extremity m. The line 

 advances into the spectnim, and at the point 

 corresponding to d, 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 A a portion of the red and green, and 

 at n the exti'eme 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 m' ri, m' ri 

 on the second prismatic image. 



The 3rd obliteration of the violet takes place at *, and that of the red at t, 

 and this corresponds in incidence with the four obliterations on the second and 

 fourth prismatic images, viz. ^ v, ^ v', m!" n'", m!" »,'", 



In all these phenomena the points m, n, ^, 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. 



