Observations and Experiments on Light. 125 



From reflecting upon these phenomena and conversing with 

 Prof. Sturtevant upon the subject, I was convinced that they 

 were to be referred to difraction, produced by the passage of hght 

 through 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 expected, as our experiments were performed upon 

 parallel rays. 



In order to understand the application 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 infiected rays^ and of those which 

 are bent outward as deflected rays, and I shall use the terms in- 

 flection and deflection in strict accordance with these definitions. 

 The plane of difraction is a plane passing through an inflected 



* See Interference in Brewster's Optics, and Herschel on Light. 



