THE INTEllFERENCE OF LIGHT. 
219 
treme rays. And the thickness of the plate being known, we may find the number 
of bands by computing the coefficient in terms of y. and X which are known. 
(23.) Writing this coefficient =q or n=qr, since here n is supposed a whole positive 
number, and q may be either + or — according to the values of /a and X, we have. 
Hence if, taking successive rays, we have always through the spectrum from the red 
to the violet, 
f H'P f^m\ ( l^p F'm\ 
or the value of q negative, the effect of the plate will be such that the arrangement 
I. will give bands : if q be positive, the effect of the plate will be such that arrange- 
ment II. will give bands. 
V ^ 
should have a maximum 
If for any combination of a plate and a medium 
or minimum value at any ray, the difference would change signs, and bands be formed 
towards that end of the spectrum where it was — with arrangement I. ; and towards 
that end where it was -j- with arrangement II. 
(24.) The general principles of the “ diffraction-theory,” as applicable to the present 
case, are precisely the same as in Mr. Airy’s paper*. But it will not be necessary 
here to go into the subject any further, since Mr. Stokes has greatly generalized 
and improved this theory so as to lead to other important results, the whole of which 
are discussed in his paper, in the present part of the Philosophical Transactions. 
Observations. 
(25.) 
Arrangement. 
Plate. 
Medium. 
Number of bands. 
Glass. 
Oil of Sassafras. 
B to D. 
D to F. 
Fto G. 
Gto H. 
Total. 
I. 
inch. 
r—-5 
•34 
•17 
•08 
•04 
•015 
No bands visible. 
Very line and close. 
Fine. 
Clear. 
Broad and clear. 
Very broad and faint. 
6 
14 
21 
24 
65 
II. 
No bands. 
(26.) 
Oil of Cassia. 
I. 
•08 
•04 
•015 
j- Too fine to count. 
Fine. 
15 
29 
32 
f Faint. 1 
1 40? / 
116 
II. 
No bands. 
'* Philosophical Transactions, 1841, Part I. 
2 G 
MDCCCXLVIII. 
