I wish to make sure that you do understand this, so let us try to illustrate it. We are in the habit of estimating things by percentage. Suppose then that we think of the light of the sun as representing one hundred points. Scientists have discovered that objects which we call yellow absorb only some twenty of these points; that, in fact, the quantity of light thrown off by what we call yellow, or in other words its value, is some eighty per cent. What we call red, however, represents some sixty per cent. of light; green, about forty per cent.