648,000 :  98 ::  4.248 to the wave-length C d.

Making the calculation, we find the wave-length for this particular kind of light to be 0.000643 of a millimeter, or 0.000026 of an inch.

FOOTNOTES: 

[Footnote 1:  Among whom may be especially mentioned the late Sir Edmund Head, Bart., with whom I had many conversations on this subject.]

[Footnote 2:  At whose hands it gives me pleasure to state I have always experienced honourable and liberal treatment.]

[Footnote 3:  One of the earliest of these came from Mr. John Amory Lowell of Boston.]

[Footnote 4:  It will be subsequently shown how this simple apparatus may be employed to determine the ‘polarizing angle’ of a liquid.]

[Footnote 5:  From this principle Sir John Herschel deduces in a simple and elegant manner the fundamental law of reflection.—­See Familiar Lectures, p. 236.]

[Footnote 6:  The low dispersive power of water masks, as Helmholtz has remarked, the imperfect achromatism of the eye.  With the naked eye I can see a distant blue disk sharply defined, but not a red one.  I can also see the lines which mark the upper and lower boundaries of a horizontally refracted spectrum sharp at the blue end, but ill-defined at the red end.  Projecting a luminous disk upon a screen, and covering one semicircle of the aperture with a red and the other with a blue or green glass, the difference between the apparent sizes of the two semicircles is in my case, and in numerous other cases, extraordinary.  Many persons, however, see the apparent sizes of the two semicircles reversed.  If with a spectacle glass I correct the dispersion of the red light over the retina, then the blue ceases to give a sharply defined image.  Thus examined, the departure of the eye from achromatism appears very gross indeed.]

[Footnote 7:  Both in foliage and in flowers there are striking differences of absorption.  The copper beech and the green beech, for example, take in different rays.  But the very growth of the tree is due to some of the rays thus taken in.  Are the chemical rays, then, the same in the copper and the green beech?  In two such flowers as the primrose and the violet, where the absorptions, to judge by the colours, are almost complementary, are the chemically active rays the same?  The general relation of colour to chemical action is worthy of the application of the method by which Dr. Draper proved so conclusively the chemical potency of the yellow rays of the sun.]

[Footnote 8:  Young, Helmholtz, and Maxwell reduce all differences of hue to combinations in different proportions of three primary colours.  It is demonstrable by experiment that from the red, green, and violet all the other colours of the spectrum may be obtained.