562 Prof. A. A. Michelson on Metallic 



have to deal with (feathers, butterfly scales, beetle wing-cases) 

 are so irregular that the quantity of " regularly " reflected 

 light which is brought to a focus by the observing telescope 

 is insignificant, and is often masked by the light diffusely 

 reflected. But by the simple device of replacing the objec- 

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

 power microscope objectives of small aperture, these difficulties 

 are so far removed that it has been possible to obtain results 

 which compare favourably with those obtained with 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 nature that they could be taken into account. 



Following is a diagram showing the results of a set of 

 measurements on a beetle having a lustre resembling burnished 

 copper. Beside it is a duplicate of the preceding observations 

 on a thin film of magenta <T1. IV. fig. 4). 



The correspondence between the two sets of curves is so 

 remarkable that it leaves no room to doubt that in this case 

 the metallic coppery colour of the wing-case is due to an 

 extremely thin film of some substance closely analogous in 

 its optical qualities to the corresponding aniline dye*. The 

 thickness of the magenta film was not very accurately deter- 

 mined, but from the fact that it was deposited from a solution 

 of 1/20 of the concentration of that which produced the cor- 

 responding thick film (whose thickness is about 0*005 mm.), 

 it is estimated that the thickness of the thin film is of the 

 order of 0*00025 mm. It is, doubtless, unsafe from this to 

 draw r any more definite conclusion regarding the film of the 

 wing-case, than to say it is probably of the same order. 



An attempt was made to check this estimate by the 

 following simple device. 



A portion of the ellipsoidal wing-case of mean radius R 

 was removed by passing over it very lightly a piece of the 

 finest emery-paper fastened to a flat piece of wood. This 

 left a clean elliptical hole of mean radius r showing the 

 edges of the " metallic" film, whose width, h, could not be 

 appreciated in a microscope with a half-inch objective. 

 If this be estimated at less than 0*001 mm. the relation 



T = tt = -4 ' gives t, the thickness of the film, less than 



h R omm. & 



a ten-thousandth of a millimetre, 



A second specimen of the same general coppery lustre, 



* The character of the curves for the organic film is considerably 

 more " metallic " than the corresponding curves for magenta. 



