ert 
4 site eee ey = Be ee oY 
126 Obs: ti and Experiments on Light. 
and a deflected ray which have diverged from the same point, 
and is always parallel to and passes through the unmodified beam 
of light. When the difracting edge isa straight line, the plane 4 
of difraction is always perpendicular to a plane passing through 
the difracting edge and the corresponding outline of its shadow. 
In an irregular or curved difracting edge the same law will hold 
with regard to any indefinitely small portions of it, which wee 
be assumed as straight lines. 
I am aware that the terms inflection and difraction are. vont as 
synonymous by many who have written upon the subject of light. 
But without the definitions and limitations, which I have just 
indicated, I should be compelled to resort to cireumlocutions, 
which might render ambiguous the explanations which I am 
about to give of the phenomena of the feather. Again, Iam not 
aware that the law which regulates the position of the plane of 
difraction has been stated by any other writer, although it is fairly 
inferrible from the facts which they have brought forward, as 
well as from experiments performed by myself, and which I hope 
to notice more fully in a subsequent communication. It will be 
seen in the sequel, that the law which regulates the position of 
the plane of difraction determines the angle, which the’ wonenes 
of colored spectra make with each other. 
Let us now turn our attention to the lattice-work formed by 
the crossing of the barbules of the feather, and inquire how the 
light passing through a single opening would be affected. The 
openings of the lattice are of course one of the four varieties of 
the parallelogram. The angles of these openings differ in the 
feathers of different birds, and in different feathers of the same 
eunant Let a bed represent one of these openings; and let us sup- 
_5 pose a beam of light passing through it perpendicular to the 
| poe [4 teas of the e-em ie is pation’ that each of the sides of 
dge; and if we take any 
two opposite sides ab, do, the leans rays ‘of one side will be on 
in the same direction as the deflected rays of the other, and» 
be liable to interfere with each other, and produce colored fringes 
pi ooh placed to receive the difracted light, and. these frin- 
Be. iediom. cacks-televod [ thexopening in a line perpen- 
dicalenball The same will be true of 
the other two sieege be, and thus we should have two rows of 
colored fringes, whose lines of direction would be perpendicular 
