Suirtevant upon the subject, I was convinced that they 
to be referred to difraction, produced by the passage of light 
igh the minute foramina formed by the crossing and. inter- 
locking of the barbules of the feather. This conviction was 
strengthened by a microscopic examination of the vane of the 
feather, which exhibited an extremely minute lattice-work be- 
tween the barbs of the feather, formed by the crossing of the 
barbules, and by noticing that the lines, in which the colored 
spectra Were arranged, were perpendicular to the bars of the lat- 
tice. The similarity between the arrangement of the colors 
in the spectra upon the screen, and those of the external frin- 
ges produced by difraction, could not fail to be observed, and to 
incline me to the opinion that the law of interference establish- 
ed by Dr. Young, had something to do with the production of 
the chromatic spectra. I was confirmed in this opinion by a 
series of experiments and measurements performed by Prof. Stur- 
tevant and myself, by which we ascertained, that corresponding 
spectra received upon a screen at different distances from the 
feather, were not arranged in straight lines, but in curves. ‘The 
curves seemed to belong to the hyperbola, and the latter to be 
formed by the section of a very acute cone. This is what might 
have been eepaanet as our eeiwene were sanscoeneia upon 
parallel rays. 
~ In order to nadeiiund she Snagdeghin: of the: law of interfer: 
ence* to the production of colored spectra by the feather, it will 
be necessary to recur to the fundamental facts of difraction. Let 
it be borne in mind, that when a beam of light falls upon the 
edge of an opaque body, the rays which pass by the edge are di- 
vided into two portions, one of which is bent into the shadow of 
the opaque body, and the other is bent outward from the body. 
‘This separation of a beam of light into two parts is called difrac- 
tion. For the sake of brevity and clearness I shall, in my sub- 
Sequent remarks, speak of those rays which are bent into the 
. shadow of the opaque body as injlected rays, and of those which 
4 are bent outward as deflected rays, and I shall use the terms in- 
3 Siection and deflection in strict accordance with these definitions. 
hte of difraction is a plane passing through an indea 
SECE gigy suas in Brewster's Optics, and Herschel on Light. 
