The sun's own light shows under this method of spectroscopic analysis millions of tints, in fact I might say millions of red tints, and so forth, right through the spectral list of colours. But also many thousands of tints are wanting. Imagine a rainbow-coloured ribbon, the colours ranged along its length, so that the ribbon is black at both ends, and that from the black of one end the colour merges into very deep red, and thence by insensible gradations through orange, yellow, green, blue, indigo, and violet, into the black of the other end. Then suppose that tens of thousands of the fine threads which run athwart the ribbon--_i.e._, the short cross threads--are drawn out. Then the ribbon, laid on a dark background showing through the spaces where the threads were drawn out, would represent the solar spectrum. We know then that the light of the sun's glowing mass either wants particular tints originally, or shines through vapours which prevent the free passage of rays of those colours. Both causes might be at work, not one only. At present we are not concerned with this particular point; but I only mention that, in reality, no tints are actually wanting, though some are very much enfeebled. The sun's light falling on any opaque object is reflected. If the object is white, the light gives exactly the same spectrum, only fainter. Thus, I take a piece of white paper on which the sun's rays are falling, and examine its light with one of Browning's spectroscopes. I get the ordinary solar spectrum. The cold white paper gives me in fact a spectrum which speaks of a heat so intense that the most stubborn metals are not merely melted but vaporized in it. But this heat resides in the sun, not in the paper. Now, speaking generally, Mars also sends us sunlight, so that when we spread out with the spectroscope the rays coming from this planet, we get the solar spectrum, only of course very much enfeebled. _But_ close examination shows that other tints besides those missing from the solar spectrum are missing from the spectrum of Mars. He reflects to us the sunlight, almost as it reaches him, but he abstracts from it a few tints on his own account.