crystallized bodies on homogeneous light. 87 
must necessarily give a result much below the truth. This 
difference is by far too great to arise from any errors of ob- 
servation ; but to obtain more exact measures, I took several 
times the apparent angular separation of the axis of each 
colour from that of the extreme red by the direct homoge- 
neous light of a sunbeam, separated by the prism, and re- 
ceived on the reflector of a divided apparatus, when, after 
the proper reductions for refraction and dispersion, the results 
were as follow : 
Colour. 
Apparent 
separation 
of the axes. 
Real 
separation 
— $a = 
Values of 
2 a . 
Number 
of obser- 
vations. 
Extreme Red. 
0 / 
0 0 
O 
/ 
0 / 
75 42 
13 
Mean Red. 
1 33 
1 
2 
73 38 
45 
Do. Orange. 
2 37 
I 
44 
72 h 
18 
Do. Yellow. 
4 0 
2 
40 
70 23 
20 
Green. 
5 49 
3 
5 2 
67 57 
16 
Blue 
8 2 
S 
21 
65 0 
*3 
Indigo. 
10 21 
6 
54 
61 54 
33 
Indigo Violet. 
11 1 7 
7 
3 1 
60 40 
2 
Mean Violet. 
13 58 
9 
17 
57 8 
2 
Extreme Violet. 
>5 23 
10 
55 i + 
8 
Though the total separation of the red and violet axes in 
this table so far exceeds what we had before estimated it at, 
I am fully satisfied that it is no way exaggerated, but rather 
falls short of the truth. It is very practicable, by combina- 
tions of coloured glasses, liquids, &c. to insulate either ex- 
tremity of the spectrum in a state of the most absolute purity. 
In this climate, the dispersed light of the sky in the neigh- 
bourhood of the sun, which always mixes with the prismatic 
beam, is so considerable as to obliterate the feeble rays which 
compose the two extremities of the spectrum, and it is only 
by interposing such combinations between the eye and the 
Iceland crystal used to analyze the polarised ray, that they 
