Che. iii 1st i'ij and Physics. 569 



upon polarized light. In brief, he determines experimentally, for 

 various angles of incidence, the ratio of the amplitudes and the 

 difference in phase of the components of the elliptic vibrations 

 resulting from reflection. The numerical data are plotted as 

 " amplitude " carves and " phase " curves. 



For sake of general orientation, the first figure contrasts the 

 curves for silver, steel, graphite, selenium, flint glass, crown glass 

 and quartz. Figure 2 relates to two typical aniline dyes, viz., fuch- 

 sine and diamond green. The third figure shows the modifications 

 in the curves produced by changing from a thick film to a very 

 thin film of magenta. The fourth and fifth figures present, side 

 by side, the curves for two specimens of " copper beetle " and thin 

 films of magenta. The correspondence between the two sets of 

 curves is so remarkable that it leaves uo room to doubt that in 

 this case the metallic coppery color of the elytra is due to an 

 extremely thin film of some substance closely analogous in its 

 optical qualities to the associated aniline dye. For the first bee- 

 tle and dye-film the thicknesses of the active layers were of the 

 order of - 00025 rnm , and for the second specimen and magenta 

 film, about - 00005 mm . Figure 6 comprises the curves for a 

 "green beetle" and for diamond green. These curves do not 

 agree very well- and, in fact, no aniline dye could be found to 

 match this beetle. Nevertheless, the curves for the insect have 

 all the characteristics pertaining to " metallic " reflection. 



The beetle wing-cases furnish in many instances a fairly smooth 

 surface, and the difficulties attendant upon obtaining the necessary 

 data are far less than when working with feathers of birds or 

 with butterfly scales. The simple device of replacing the objec- 

 tive of the collimator and of the observing telescope by low-power 

 microscope objectives of small apertures removed these difficulties 

 so far as to make it possible to obtain results which compare 

 favorably with those found for the aniline films. "In some of 

 the measurements it has been found possible to deal with a single 

 butterfly scale ; and in these the irregularities of the surface were 

 often insignificant, or of such a nature that they could be taken 

 into account." 



Figure 1 gives the curves for the green of a peacock feather, 

 for a green humming-bird, for a red humming-bird, for a blue 

 humming-bird, and for the blue-winged butterfly Morpho alga. 

 For all the feathers, except those belonging to the red humming- 

 bird, the optical data were of such a nature as to lead to the con- 

 clusion that the film which gives rise to the surface color is 

 extremely thin. 



Michelson remarks : "The total number of specimens which 

 have been examined is perhaps not so large as it should be 

 to draw general conclusions, and it is clearly desirable that it 

 be extended ; but so far the evidence for surface film, as the 

 effective source of the metallic colors in birds and insects, is 

 entirely conclusive." In this connection, reference should also 

 be made to the essay of Dr. B. Walter entitled " Die Oberflachen- 

 oder Schiller Farben." 



