THE INTERFERENCE OF EIGHT. 
22 i 
Or the same result might perhaps be deduced more directly from considering that 
as O is polarized parallel, and E perpendicular to the principal section, and that the 
short diagonal of the Nicol-prisrn is horizontal when light polarized by reflexion from 
an horizontal surface is transmitted, — or that the short diagonal is perpendicular to 
the plane of polarization of the transmitted ray. Then since when the short diagonal 
is perpendicular to the principal section of the calc-spar, the fine bands are stopped 
and the broad transmitted, it follows that the broad bands are polarized parallel to 
the principal section, or belong to O. 
(32.) 
Arrangement. 
Plate. 
Medium. 
Number of bands. 
Quartz cut perpen- 
dicular to axis. 
Oil of Sassafras. 
B to D. 
D to F. 
Fto G. 
G to H. 
I. 
inch. 
t = -15 
Position vertical. 
f”Bands fine and clear, "1 
<( broader towards red )> 
(_ end J 
8 
43 
Too fine 
estima 
70? 
to count ; 
ted at 
80 ? 
Position 1 
Position 2 
1 Bands rather finer. 
With the plate inclined in position 1, eight inteiwals of indistinctness (extending 
over four or five bands each) occur from H to about E ; from E to D the bands 
altogether become very faint : perhaps two such intervals may be discerned : from 
D to B no bands appear. On applying a Nicol-prism, the intervals of indistinctness 
disappear at each quarter of a revolution. 
Calculation. 
(33.) In all the following calculations for n by the formula (22.), the values em- 
ployed for the reciprocals of the wave-lengths of primary rays are as follows ; — 
Ray. 
X (decimals of 1 French 
inch). 
1 
X 
B. 
•00002541 
39354 
D. 
•00002175 
45977 
E. 
•00001945 
51413 
F. 
•00001794 
55741 
G. 
•00001587 
63011 
H. 
•00001464 
68306 
The values of the term ^ are interesting as expressing the absolute inter- 
val of route in wave-lengths of the two interfering rays, and thus, when compared 
with observation, conveying an idea of the extent to which the regularity of the 
undulations is kept up. 
The indices here used are those contained in my “ Report on Refractive Indices”*. 
For glass I have assumed the indices of Fraunhofer’s crown glass. No. 9. 
* British Association Report, 1839. 
2 G 2 
