238 
MR. AIRY ON THE THEORETICAL EXPLANATION OP 
is least for the most refrangible rays. Moreover, R is greatest, or — R is least, for 
the most refrangible rays. Hence the effect of the addition of the term — R is 
to make the variation of the argument of the cosine still more rapid for the variation 
of X, and the positive and negative values of the cosine will therefore destroy each 
other, or the third term may be neglected. 
The expression for intensity is, therefore, reduced to its first term 2, or there are 
no visible bands in the spectrum. 
Secondly , suppose the blue end of the spectrum to be on the same side as the plate 
of mica. 
The second term of the expression may be neglected, as before. But with regard 
to the third term, the circumstances are entirely different. For k is now least for the 
most refrangible rays (the blue end of the spectrum formed on the retina being on 
the side opposite to the mica), and therefore <p ~ d - g — is greatest 
for the most refrangible rays ; and therefore the chromatic variations of the different 
parts of the argument <p \^\J - • — + g — —j — R have a tendency to destroy 
each other. And by proper selection of the thickness of the piece of mica, the chro- 
matic variations of R (for the colours which fall upon the same point of the retina) 
may be the same as the chromatic variations of <p (V^-v + s-v)’ forthose 
values of the function which make the bands most brilliant. In this case, then, the 
bands produced by all the neighbouring colours will be aggregate in intensity, and 
therefore strong bands will be seen on the spectrum. 
With regard to the place at which any bright or dark band is seen, as depending 
on the place of the edge of the mica ; that is, with regard to the value of l for one of 
these bands, as depending on g ; it will be evident that the intensity (whether strongest 
c l 
or weakest) will be preserved the same by keeping j - g the same ; that is, there 
d CT 
will be a band of the same character so long as l varies in the same degree as — , but 
in the opposite direction ; that is, the band upon the retina will shift in the direction 
opposite to the shift of the mica, or will appear to the mind to shift in the same di- 
rection as the mica. But this shift will be small when a is small. 
3. Suppose that the eye is too near to see the lines of colour distinctly, and that 
other circumstances are the same as in the second problem. 
The expression for intensity in this case is 
2 — G 
cl 
cos <p 
L 2 a 1 
\c e 
+ g (a 
