penetrating its uppermost layer.  Thus, if dyed fabrics are examined by the spectroscope, the same appearances are generally observed as with the solution of the corresponding colouring matters.  An absorption spectrum is in each case obtained, but the one from the solution is the purer, for it does not contain the mixed white light reflected from the surfaces of coloured objects.  Let us now take an example.  We will take a cylinder glass full of picric acid in water, and of a yellow colour.  Now when I pass white light through that solution and examine the emerging light, which looks, to my naked eye, yellow, I find by the spectroscope that what has taken place is this:  the blue part of the spectrum is totally extinguished as far as G and 2/3 of F. That is all.  Then why, say you, does that liquid look yellow if all the rest of those rays pass through and enter the eye, namely, the blue-green with a trifle of blue, the green, yellow, orange, and red?  The reason is this:  we have already seen that the colours complementary to, and so producing white light with red, are green and greenish-blue or bluish-green.  Hence these cancel, so to say, and we only see yellow.  We do not see a pure yellow, then, in picric acid, but yellow with a considerable amount of white.  Here is a piece of scarlet paper.  Why does it appear scarlet?  Because from the white light falling upon it, it practically absorbs all the rays of the spectrum except the red and orange ones, and these it reflects.  If this be so, then, and we take our spectrum band of perfectly pure colours and pass our strip of scarlet paper along that variously coloured band of light, we shall be able to test the truth of several statements I have made as to the nature of colour.  I have said colour is only an impression, and not a reality; and that it does not exist apart from light.  Now, I can show you more, namely, that the colour of the so-called coloured object is entirely dependent on the existence in the light of the special coloured rays which it radiates, and that this scarlet paper depends on the red light of the spectrum for the existence of its redness.  On passing the piece of scarlet paper along the coloured band of light, it appears red only when in the red portion of the spectrum, whilst in the other portions, though it is illumined, yet it has no colour, in fact it looks black.  Hence what I have said is true, and, moreover, that red paper looks red because, as you see, it absorbs and extinguishes all the rays of the spectrum but the red ones, and these it radiates.  A bright green strip of paper placed in the red has no colour, and looks black, but transferred to the pure green portion it radiates that at once, does not absorb it as it did the red, and so the green shines out finely.  I have told you that sodium salts give to a colourless flame a fine monochromatic or pure yellow colour.  Now, if this be so, and if all the light available in this world were of such a character, then such a colour as blue would be unknown.