~ oa My 
ie Daneel to. the parallel sides of the opening, and consequently 
2, ; 
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a ees 
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ae 4 
Observations and Experiments on Light. 127 
crossing each other at angles equal to those of the opening. But 
a part of the light would pass through the centre of the opening 
unbent, and would form upon the screen a white i image at the in- 
tersecting point of the two rows of colored fringes. Thus it 
will be seen, that a beam of light passing through a single open- 
ing of the kind above described, would be divided into nine parts, 
four being produced by the inflection of the four sides, four more 
by the deflection of the same, and one being the remains of the 
beam that pass on unmodified. Now let us suppose that a beam 
of light, instead of passing through a single opening, passes 
through an extremely minute lattice, containing an indefinite 
number of such openings, as in the case of the feather. As all 
the bars of the lattice are parallel respectively to those which sur- 
round each individual opening, it is evident that the general ef- 
fect upon the beam will be the same as that of a single opening, 
with this difference, that the range within which interference 
would take place, would be greatly enlarged, by enabling the in- 
flected and deflected rays from different openings to interfere with 
each other; and thus the fringes, which are scarcely perceptible, 
when formed by a single opening or a single edge, become bril- 
liant spectra, when a beam of light. is passed through a lattice of 
the kind described. ll this is realized in the experiments with 
the feather. It is proper to remark, however, that the central 
white. image is probably not formed entirely of unmodified light, 
but is partly produced by light slightly inflected by the opposite 
edges of the bars of the lattice, and corresponding with the in- 
ternal fringes, first explained by Dr. Young upon the principle of 
interference. It is not improbable, that some of the deflected 
rays fall within the central white image and add to its brightness. 
The faint spectra in the angular spaces may be explained by sup- 
posing that they are formed by light, which has undergone two 
inflections or two deflections, or one inflection and one deflection, 
by two contiguous bars of the lattice. 
- It should be noticed here, that all the colored spectra, as well as 
the central white one, are considerably elongated in a direction 
perpendicular to the barbs of the feather. With a very delicate 
feather and a small luminous object, the eye can easily distinguish 
emis of colored — arranged in the same direction. This is 
1. 
ght have been expected, and gives us some idea of the ef- 
