226 
MR. AIRY ON THE THEORETICAL EXPLANATION OF 
I mention these matters merely for the purpose of showing distinctly that the obser- 
vation of the facts was with me long anterior to the explanation, and that therefore 
no leaning in favour of the theory can have warped iny judgment in the perception 
of experimental facts not wholly coinciding with Sir David Brewster’s. Indeed, I 
should never have thought of opposing my evidence on a matter of observation to 
that of the Father of Modern Experimental Optics, unless the reasons for disagree- 
ment were not only, in my judgment, well-grounded, but also unmingled and inde- 
pendent. 
The facts, then, as I have observed them, are as follows : 
1. When a prismatic spectrum is viewed, out of focus, bands are formed by placing 
a piece of mica of a proper thickness (indefinite within wide limits) to cover 
that half of the pupil of the eye, which is on the same side as the violet end of the 
spectrum. 
2. No bands are formed with any thickness of mica if it is placed on the same side 
as the red end of the spectrum. 
3. When the eye is too far off to see the spectrum distinctly ; then, upon passing 
the mica before the eye from the violet end of the spectrum, bands are seen advancing 
in the same direction over the spectrum. 
4. When the eye is too near to the spectrum to see it distinctly; then, upon pass- 
ing the mica from the violet end of the spectrum, bands are seen advancing in the 
opposite direction over the spectrum. 
5. If the eye be so far off, and the spectrum in consequence seen so indistinctly, 
that the ruddy portions are nearly mingled with the blue ; upon applying the mica 
to cover half the pupil on the side corresponding to the violet end of the spectrum, 
bands are seen very well defined, but narrow. 
6. If the eye and mica approach to the position of distinct vision of the spectrum, 
the bands become somewhat broader, and near the position of distinct vision some- 
times disappear. On still approaching to the spectrum, the bands reappear and be- 
come narrower, but are not, I think, seen so distinctly as when the eye is too far off 
for distinct vision. I know not, however, whether this may depend upon the prac- 
tical difficulty of that part of the experiment. 
7. Bands which are visible when the mica is on the violet side and invisible when 
it is on the red side, never occur when the spectrum is pure. 
8. Bands are frequently visible when the spectrum is pure ; but in that case they 
can be seen equally well, whether the mica be advanced from the violet side or from 
the red side. 
The bands mentioned in No. 8. have been long since observed and explained by 
Mr. Talbot*, and of these therefore I shall make no further mention. 
The general outline of the explanation of the bands seen when the spectrum is out 
of focus, is as follows : 
* London and Edinburgh Philosophical Magazine, vol. x. p. 364. 
