128 Observations and Experiments on Light. 



feet produced by passing a beam of light between extremely minute 

 parallel bars arranged in the same plane very close to each other. 

 The sun, moon, stars, the flame of a lamp, a small aperture in a 

 dark room, &c., are convenient objects to be examined with the 

 eye through the vane of a feather. When we wish to examine 

 a kiminous object through the vane of a feather, one of a dull or 

 dark color should be chosen, as a white feather transmits so much 

 hght, as soon to exhaust the sensibility of the retina. For form- 

 ing colored spectra on a screen a white feather is preferable. 

 Those feathers taken from the wing and tail, whose vanes ap- 

 proach the nearest to a plane, give the most regular arrangement 

 of the spectra. The feathers of small birds, from the greater 

 minuteness and delicacy of their structure, produce the most bril- 

 hant and extensive colors. We see here the same principle, 

 which Dr. Young applied to the construction of the Eriameter.* 

 In looking through the vane of a feather at a bright object, the 

 most brilliant spectra are seen on the side towards the outer edge 

 of the feather. This may be owing to the thinning out of the 

 feather towards the edge. 



If the above explanation of the phenomena of the feather be 

 correct, it follows, that if an opaque screen be perforated with 

 circular holes sufficiently minute and near to each other, it would 

 produce a succession of colored rings. When a beam of light 

 passes through a lock of cotton, wool, or raw silk, inflection and 

 deflection will take place in every possible direction, producing a 

 blending of all the colors into white light in the centre, and a 

 succession of colored rings in receding from the centre. As in 

 the feather there is a regular arrangement of the difracting fibres, 

 there is a corresponding arrangement of the colored spectra. The 

 explanation of the colored rings produced by transmitting a beam 

 of light through a lock of cotton, &c., applies to those produced 

 by transmitting a beam through a plate of glass covered with fine 

 particles. 



Having satisfied myself with regard to the structure of the 

 vane of the feather, and the mode in which it operates in produ- 

 cing colored spectra, I concluded, that, if that structure could be 

 imitated by any artificial contrivance, the same effects might be 

 produced as by the feather. I shall not detain the reader by de- 



" See Brewster's Optics. 



