plate to pass, but in addition it reflects to the eye a beam of light from a third slip of colored paper at V; and, by revolving the second glass plate slightly, the intensity of the third beam is easily regulated. This arrangement can be used to produce white light, by the mixture of three colors, for example, vermilion, emerald-green, and the violet just mentioned.
Let us pass, in the next place, to the consideration of another class of facts, which have an important bearing on our subject. If you illuminate some such object as a sheet of paper with a very moderate light, then, upon doubling the amount of light falling on it, it is possible that the paper, in the second case, may appear to you twice as bright as it did at first. But, if this process be for some time continued, you will soon come to a point where doubling the actual illumination produces very little effect, and finally a stage will be reached where a very great increase of actual illumination produces no additional effect on the eye at all, your paper looking no brighter than in a much feebler light. Let me make an experiment, to at least partially illustrate this: We have now upon the screen four large squares of white light, and they are, as you see, all of equal brightness. But, by turning this Iceland-spar prism, I superimpose one of the squares upon its neighbor; the central square now seems rather brighter than its companions, but I think no one in this room would suspect that its actual illumination was twice as great as that of the others. To take a still more striking example out of your own experience: you have often noticed the reflection of the gas-flames in the streets against the four panes of glass used to protect them, and have seen that the real flame looks brighter than the reflected one; but who would suppose that its actual luminosity was more than eleven times greater than that of its companion? In point of fact, sensation does not, for the most part, increase as rapidly as the actual intensity of the light exciting it, and a point can finally be reached where sensation does not increase at all, even though the actual brightness of the light is greatly multiplied. Our nervous organization is, in this direction, limited and finite, just as it is in all others.
The next matter to which your attention is called is really allied