Observations and Experiments on Light. 129 



tailing all the expedients which I resorted to in the subsequent 

 course of my experiments, but will endeavor to indicate some 

 modes of imitating the structure and effects of the vane of the 

 feather more perfectly than can be done by any means which are 

 at my command. Let it suffice to say, in the mean time, that 

 silk cloth of a close and delicate texture, a dense gauze of fine 

 wire, and similar contrivances, answer as clumsy substitutes for 

 the vane of the feather. 



The difficulty of obtaining the necessary materials, and of 

 commanding the requisite mechanical skill, has prevented me 

 from executing the most desirable plans, that have presented 

 themselves to my mind. A convenient mode of arranging paral- 

 lel fibres is, to bend a steel wire thus, C and wind a fine 

 silk thread or delicate wire across its parallel sides. With a con- 

 trivance of this kind I was able to produce a row of spectra or 

 fringes in a line perpendicular to the parallel fibres. I made use 

 of a fine silk thread, but it is manifest, that fibres more minute, 

 skilfully arranged, would greatly increase the brilliancy of the 

 phenomena. In this case, it will be seen that the rays undergo 

 two difractions in the same plane. The second set of fibres 

 would increase or diminish the effect of the first set, according as 

 its difracting influence coincides with, or counteracts that of the 

 first ; and it is probable, that both of these effects are produced 

 upon different rays. A preferable constrirction would be to take 

 a rectangular metallic frame and wind the finest platinum wire 

 across two of its parallel sides, so close as just to admit the pas- 

 sage of light between the parallel turns of the wire. The wire 

 may be fastened by metallic bars screwed down upon it, where it 

 crosses the exterior sides of the frame, and then one set of the 

 parallel turns of the wire may be cut away, so as to leave only 

 one set to act upon a transmitted beam of light. Two of these 

 contrivances might be placed together and turned upon each other, 

 so that the parallel wires in one could be made to cross those of 

 the other at any convenient angle, and thus the phenomena of 

 the feather would be imitated. The crossing of the wires might 

 be secured by winding the same frame in opposite directions, fas- 

 tening the wires and cutting them away on one side in the man- 

 ner above described. The first method, however, is preferable, 

 as it admits the change at pleasure of the angles at which the 

 two sets of parallel wires cross each other. This apparatus 



Vol. xLii, No. 1.— Oct.-Dec. 1841. 17 



