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THE POPULAR SCIENCE MONTHLY. 



sent to the eye more light, but it will appear paler ; the color-element 

 will begin to be pushed into the background. Conversely, if we now 

 should subject our mixture of white and colored light to analysis by 

 a second spectroscope, we should infallibly detect the presence of the 

 white as well as of the colored light; or, if no white light were pres- 

 ent, that would also be equally apparent. 



Fig. \. is the observing-telescope ; S the scale-telescope ; L the source of light which furnishes 

 the spectrum ; W the white light which is projected on the spectrum. 



Taking all this into consideration, it is evident that when a par- 

 ticular color is presented to us we can affirm that it is perfectly pure, 

 viz., entirely free from white light ; or that it contains mingled with 

 it a larger or smaller proportion of this foreign element. This fur- 

 nishes us with our first clew toward a classification of colors : our 

 pure standard colors are to be those found in the spectrum ; the col- 

 ored light coming from the surfaces of natural objects, or from painted 

 surfaces, we must compare with the tints of the spectrum. If this is 

 done, in almost every case the presence of more or less white light will 

 be detected ; in the great majority of instances its preponderance 

 over the colored light will be found quite marked. To illustrate by 

 an example : If white paper be painted with vermilion, and compared 

 with a solar spectrum, it will be found that it corresponds in general 

 tone with a certain portion of the red space; but the two colors never 

 match perfectly, that from the paper always appearing too pale. If, 

 now, white light be added to the pure spectral tint, by reflecting a 

 small amount of it into the observing-telescope, it will become possi- 

 ble to match the two colors, and, if we know what proportion of white 

 light has been added, we can afterward say that the light reflected 



