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white light. If the greater part of the incident white light 
were in the same way, decomposed, the metals instead of 
appearing to us white, would shine with splendid colors. 
The colors of natural objects are always mixed with white 
light, that is, the white light falling on the surfaces of 
natural objects is never completely decomposed, and in this 
regard the case of the metals shows a difference in degree 
and not in kind. To the eye of the scientist then, all the 
metals may be colored; the colors are ordinarily invisible 
simply because they are diluted or overpowered by the 
white light with which they are mingled. With the 
explanation thus given I assume in this paper that all metals 
are colored. 
The most satisfactory method of rendering the colors of 
metals apparent, heretofore proposed, consists in repeatedly 
reflecting a beam of white light from the metallic surface 
under examination. A convenient arrangement is two 
parallel plates of the metal between which the light is 
reflected from one to the other at a small angle of incidence. 
At each incidence the white light is partially decomposed 
and if the number of incidences be sufficiently multiplied, all 
the white light will have disappeared, and only the pure 
colored rays be visible. In this way the colors of most of 
the metals have been exactly determined. The actual 
experiment, however, is not a very brilliant one, inasmuch as 
the larger part of the light with which it begins is lost by 
gradual diffusion; especially the colored light is so lost, 
probably for the reason that the decomposition of the white 
light takes place within the reflecting surface. 
Another method of developing the true colors of metals, 
has recently occurred to me, and it is the main purpose of 
this paper to describe it. I present first a few theoretical 
considerations. 
When white light is decomposed by a colored body, the 
reflected colored ray is complementary to that part of the 
white light which is transmitted or absorbed; if a colored. 
body be seen both by reflected and transmitted light, the 
colors so seen should ‘be complementary, or an approach to 
