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 different 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 different 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 
Yellow. 
Pink. 
90 0 
1st junction of pink and blue 76 20 
Greenish blue. 
Yellow. 
