periodical colours produced by grooved surfaces. 51 



Such being the phenomena exJiibited by the ordinary image 

 formed by reflexion from the original spaces w, I now pro- 

 ceeded to examine the prismatic images in the first specimen 

 with 1000 grooves, and I observed the following appearances. 



Let A B, Fig. 9, Plate II. be the reflected image of the regr 

 tangular aperture from the spaces n, and a 6, a' 6', a" b'\ a" 

 h"\ the prismatic images of it, v v^ v' v'., &c. being the violet 

 sides, and r r, r' ?-', &c. the red sides of these spectra. Then 

 in the 



1st spectrum ah, the violet rays are obliterated at m at an 

 incidence of 74°, and the red rays at n at an incidence of ^6°, 

 the intermediate colours, blue green, being obhterated at in- 

 termediate points between m and n, and at angles of incideijce 

 intermediate between 74° and Q{i°. In the 



2d spectrum a' b', the violet rays are obliterated at m! at 

 an incidence of QQ^ 20'', and the red at n at 55)° 45'. In the 



3d spectrum a" h'\ the violet rays are obliterated at m" at 

 57% and the red at n" at 41° 35'. And in the 



4th spectrum a'" 6'", the violet rays are obliterated at m'^' 

 at 48°, and the red rays at n'" at 23° m. 



Another similar succession of obliterated tints takes place 

 on all the prismatic images at a lesser incidence, as shown at 

 ^v (jJ v' the violet being obliterated at /a, and the red at v, and 

 the intermediate colours at intermediate points. In this second 

 succession the line [m v begins and ends at the same angle of in- 

 cidence, as the line m" n" in the third prismatic image a" b' ; 

 and the line // / on the second prismatic image corresponds with 

 m'" ril" on the fourth prismatic image. 



This sin ovular obliteration of the colours is shown more clear- 

 ly in Fig. 10, where r mvn'is a part of one of the prismatic 

 images, r v the red space, g g the green space, b b the blue, 

 and V V the violet space. The line of obliteration m n in be- 

 ginning at m obliterates the extreme violet at m ; so that the 

 curve of illumination abm. Fig. 11, is just affected at one ex- 

 tremity m. The line advances into the spectrum, and at the 

 point corresponding to d, Fig. 10, a portion of the blue and 

 violet is obliterated, as shown by the nptch in the curve ; at e 

 a portion of the green and blue ; at ^ a portion of the red and 

 green, an4 at n the extreme red. 



