236 
MR. AIRY ON THE THEORETICAL EXPLANATION OF 
the weak light of another ; and thus the alternations of light and dark will be visible 
only near the origins of s. Thirdly, that if the origins of ,<? in the non-interfering 
streams of light do coincide, but if the values of X are very different, the values of 6' 
for the same value of b (expressed by \J ^ -J - gj may, when b is large, differ 
very much for the different values of X, and thus the strong light of one stream will 
be mingled with the weak light of another : and here also the alternation of light and 
dark will be visible only near the origins of s. I will now proceed with practical ap- 
plications of our formulae. 
1. Suppose that heterogeneous light issues from a point or a narrow line, and is 
viewed by an eye too distant to see it distinctly : a thin plate of mica, with its edge 
parallel to the line, is gradually brought across the pupil of the eye : to describe the 
appearance of the line. 
The formula is 
2 — 
G 
(V; 
2 a 
Ace 
cb , \ 
• — + gj .COS <p 
and the aggregate of all the values of this expression for the different values of X is to 
be taken. Now the last term may be rejected at once. For the values of R will 
vary very much, perhaps to the extent of many multiples of 360°, for the differently 
coloured rays ; and therefore, in the aggregate, the expressions 
which are added together, will have had all values, positive and negative, 
pression is therefore 
2 ~ G + e) + 
The ex- 
The alternations of light represented by this, for the reasons above mentioned, will 
be sensible only where -}-,§• is small, that is, where b differs little from — If 
then the edge of the mica is on the right side of the centre of the pupil, the bands of 
light will be on the left side of the centre of the confused image on the retina, or will 
give to the mind the perception of bands of light on the right side of the object viewed : 
and therefore, upon moving the plate of mica, the bands will appear to move in the 
same direction. Moreover, the band which is most conspicuous, because its coeffi- 
cient of the variable part is largest, because it is unaffected by the variation of X, and 
c h 
because the other bands are ranged symmetrically on both sides, is that for which — 
-J- ^ is 0, and for this the expression of intensity for every colour is 2 — G (0) cos <p (0) 
= 2—1 = 1, which is its minimum value. Hence there will be seen a central dark 
