Prof. Powell: Comparison of Refractive Indices with Theory. 465 
The values of the wave-length constants A and B for each ray, 
independent of the medium, are taken from my Treatise (Undulatory 
Theory applied to Dispersion, &c., Art. 270). Combining these, we 
obtain AD and BD’ for each ray in the medium. 
Thence Sir W. R. Hamilton’s formula (76. Art. 237) gives for any 
ray, p=ppt (AD+ BD’) ; 
the upper sign being used for rays above F, the lower for those below. 
Ether.—Dale and Gladstone. 
y. 
Ray. SSS Difference. 
Observation. Theory. 
B 1°3545 
C 1°3554 1°3544 —‘0010 
D 1°3566 1°3566 —0000 
E 1°3590 1°3586 —‘0004 
F 1°3606 
e 1-3646 1:3646 -0000 
H 1:3683 
Hydrate of Phenyle.—Dale and Gladstone. 
B 15416 | i 
Cc 15433 | 1°5428 —*0005 
D 15488 | 1°5495 +°0007 
E 1°5564 | 1°5567 +°0003 
F 1°5639 
G 1:5763 1°5772 +0009 
H 1°5886 
In both these media, of low dispersive and refractive power, the 
accordances of theory and observation are sufficiently close. 
Oil of Lavender.—Powell. 
aris pohly Lb t.464]---ce he ene 
c 14658 | 1:4632 —-0026 
D 1°4660 1:4678 +:0018 
E 1-4728 | 1°4726 —0002 
F 14760 | 
G 1:4837 1°4848 +0011 
H 1°4930? | 
Oil of Sandal-wood.—Powell. 
B 1°5034 | 
y 1°5058 | 1:4988 —-0070 
| D 1°5091 1°5062 —°0029 
E 15117. | ~=—- 15102 —0015 
F 15151 
Gin b> Wehzal 15271 +°0040 
i pate Nee Pee mee 8 RO Pre ie Pelt Oe 1°5398? 
