periodical colours produced by grooved surfaces. 53 



at a greater angle of incidence ; and other two rows of minima, 

 namely, rows of five and six, would be found extending to the 

 fifth and sixth prismatic images. The arrangement and suc- 

 cession of these is easily deducible from Fig. 11, where the law 

 of the phenomenon is obvious to the eye. 



The following table contains the angles of incidence reckon- 

 ed from the perpendicular at which these minima occur in the 

 extreme rays. 



Position of the minima in red light. 



Ord. Im. 1. Pris. Im. 2. Pris. Im. 3. Pris. Im. 4. Pris. Im. 



First minima, /?, 76 66 55 45 41 35 23 30 



Secoml minima r, 55 45 41 35 23 30 



Third minima, 23 30 



Position of the minima in violet light. 



First minima, 81 30 74 66 20 57 48 



Second minima, 66 20 57 48 



Third minima, 48 



When the steel with 1000 grooves is exposed to common 

 light, and the incident ray is very near the perpendicular, the 

 5th, 6th, 7th, and 8th prismatic images are combined into a 

 mass of whitish light terminated externally by a black space. 

 As the angle of incidence increases, the 6th, 7th, 8th, and 9th 

 images are combined into this mass, then the 7th, 8th, 9th, 

 and 10th images, and so on, the black space which terminates 

 this mass receding from the axis or image A B, Fig. 10, as the 

 obliquity of the incident ray increases. 



Having covered the steel plate with water and oil of cassia 

 in succession, I found the angular distances of the black space 

 to be as follows at the same incidence. 



o / , 



Air, - - - 12 23 



Water, - - - 17 15 

 Oil of cassia, - - 21 22 

 The sines of which are inversely as the indices of refraction 

 of the fluids. 



Phenomena analogous to those above described take place 

 on the grooved surfaces of gold, silver, and calcareous spar, &c. 



