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THE POPULAR SCIENCE MONTHLY. 



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. 



Fig 







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 illu- 

 minate 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 possi- 

 ble 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 con- 

 tinued, you will soon come to a point where doubling the actual illu- 

 mination produces very little effect, and finally a stage will be reached 

 where a very great increase of actual illumination produces no addi- 

 tional 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 ex- 

 citing 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 



