308 DR. BREWSTER ON PERIODICAL COLOURS PRODUCED BY THE 

 Position of the minima in violet light. 



Ord. Im. 1st Prism. Im. 2nd Prism. Im. 3rd Prism. Im. 4th Prism. Im. 



First minima 81 30 7°4 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 in- 

 cident 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 AB, Fig. 1, 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. 



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. 



In order to study this subject under a more general aspect, I was desirous 

 of examining the phenomena exhibited by grooved surfaces of diffferent refrac- 

 tive powers. It was obviously impossible to procure systems of lines upon 

 transparent bodies in which the grooves should have exactly the same distance 

 and magnitude ; but I conceived it practicable to impress upon diffferent sub- 

 stances the very grooves which produced the preceding phenomena, and I suc- 

 ceeded in impressing the system of 1000 grooves upon tin, realgar, and isinglass. 



The following results were obtained with Tin, the colours being those upon 



AB, Fig. 1. 



White ... 90 



Yellow. 

 Pink. 



o 



1st junction of pink and blue 76 20 



Greenish blue. 



Yellow. 



