This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Poets have occasionally said harsh things of science, - indeed one goes so far as to stigmatise the man of science as one who would untwist the rainbow, and even botanise upon his mother's grave. Still we are in duty bound to listen to scientific conclusions, although they go against the opinions and associations of the past or present, and pull down much that we cling to with true conservative spirit.
The able author of 'Inventions of the Nineteenth Century' takes up a Rose and thus discourses upon it. The Rose, he says, is red - not because it has redness in itself, but because the light which falls upon it contains some rays in which there are movements that occur just the number of times per second that gives us the impression we call redness, - in short, the colour comes not from the flower, but from the light. Now, might we not say the Rose is always red by whatever light we see it, and therefore the colour must be in the flower 1 For whether we view it by sunlight or moonlight, or candle-light or gas-light, we invariably see that it is red. All this must be granted, yet it is precisely this circumstance, the seemingly invariable association of the object with a certain impression (in this case redness), that leads our judgment astray, and makes us believe that the colour is in the object; and it certainly requires the comparison of many observations and experiments to establish a truth so unlike the settled convictions of early life. The point in question, however, is one extremely easy of experiment, and one which would form a source of pleasant entertainment for any spare winter's evening.
Let us procure a spirit-lamp, and place on the wick a piece of common salt about as large as a pea; let the lamp be lighted in a room from which all other light is completely excluded, and bring near to the flame a red Rose or scarlet Geranium. The flowers will be seen to have strangely transformed from the brightness of their colours, to what appears an ashy grey. A ball of bright scarlet wool, such as ladies use to work their bright designs, when held near this flame is apparently no longer scarlet, but of the more homely grey. The experiment may be made even more striking when at a little distance from the spirit-lamp is placed a feeble light of the ordinary kind - a rush-light, for example.
A bouquet viewed by the rush-light shows the so - called natural colours of the flowers; but when brought under the salted light or flame, the Roses, Verbenas, and Larkspurs, or whatever else the bouquet may contain, with one exception - viz., that of yellow flowers - all become of a uniform ashy grey. The same influence is observable upon fleshy matter - the pink coral-like lips become a livid hue, almost repulsive to weak nerves. Let us now seek for a few explanations. In the first place, it may be stated that spirit burnt in the way indicated gives off little or no light of any kind. When, however, common salt is introduced into the flame, then light is given off; but light of only one particular colour - that colour being yellow. Our search would be in vain for red, green, blue, or violet vibrations; and as the objects on which the light falls cannot supply these, it follows that with this light no impression corresponding to these colours can be produced on the eye, whatever may be the object upon which it falls. We must therefore come to the conclusion that the colours come from the light, rather than from the object.
Of course it must be remembered that there is in each substance something that determines which are the rays absorbed, and which are the rays reflected to the eye; something that can destroy certain waves but is powerless over others, that rebound from the substance, and reaching the eye, there produce their characteristic impression. And it is but this power of sending back only certain rays among the multitude which a sunbeam furnishes, that can be attributed to objects, when we may properly say that flowers have such and such a colour. In this sense only, then, we have a right to say that the Rose is red. Yet it is also true that the redness is not in the Rose, if we believe that the agent which produces in our visual organs the impression of colour is not in the objects, but in the light which falls upon them. We offer the above as a mere outline of the subject; and as gardeners, like other men, require some recreative study which unites pleasure with profit, we think there could not be a better than this science of colour, which has a rare capacity for adding both to our professional and intellectual enjoyment.