TJie Structure of Coloured Bodies. 209 



results of sufficient importance to justify the expenditure of 

 the time and trouble required ? The answer to this may- 

 best be given in the form of an example. For this purpose 

 any of the various bodies we have been treating of might 

 be selected. I have taken an iridescent feather from the 

 breast of the humming-bird Chrysolampis mosquitus. Placed 

 on the stage of the microscope, and examined in the ordinary 

 way, it is seen to consist of a central shaft or rachis, from 

 the sides of which spring the so-called barbs, and arranged 

 along the edge of these are seen numbers of elongated 

 flat bodies, termed 'barbules,' which, towards the extremity 

 of the feather, overlap. These latter, with the barbs, con- 

 stitute the web, and the two webs with the intermediate 

 shaft, the vane of the feather. These barbules are seen to 

 be brilliantly coloured, they constitute the colour producing 

 structure of this iridescent feather. Here, so far as the struc- 

 ture of these barbules goes, microscopic examination ends, 

 and here ultra-microscopic investigation steps in. Retain- 

 ing the structure on the stage of the microscope, modified 

 so as to permit of the necessary adjustment, the object 

 is, by suitable movements of stage and illumination, ex- 

 amined by light falling on it, and reflected from it at varying 

 angles. It is seen to change colour from a higher towards 

 a lower order of tint as the incident light becomes more 

 and more oblique ; in other words, it belongs to the class 

 of iridescent bodies. Such colours might be due to disper- 

 sion, polarisation, diffraction, or interference of thin plates. 

 Polarisation we may at once exclude, since the object is a 

 natural body, and colour by polarisation, so far as I am 

 aware, is unknown in nature. Examined according to 

 the method already alluded to for the distinguishing 

 of diffraction colours from those of thin plates, it is 

 seen to belong to this latter group, to consist of 

 thin plates. But the theory of colour-production by thin 

 plates is well understood, and it can easily be shown that, 

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