THE EYE AND THE PHYSIOLOGY OF VISION. 537 



who picks out of the sands the yellow gold, but allows 

 the white sands to be washed through his fingers, so this 

 window pane picks out all the colors of the spectrum, while 

 the red color, like the sand in the analogy, it allows to pass 

 through. We can easily understand how a boy looking 

 over an assortment of marbles would pick out for his own 

 use and put into his pocket all those of certain desirable 

 colors and reject possibly all those of some, to him, unsatis- 

 factory color. So this window pane picks out and absorbs 

 from the composition of colors which reach it in the white 

 light certain colors, while it rejects, that is, allows to pass 

 without hindrance, the red light which falling into the eye 

 finally arouses there the corresponding sensation. It is evi. 

 dent that such a window pane could look red only when red 

 light reached it, since that is the only light it allows to pass 

 through. If blue light alone should reach it the blue would 

 be entirely absorbed and the result would be no light at all 

 traversing it, the window pane would look black. One may 

 easily try the experiment of looking through a piece of red 

 glass at an intensely blue object. The red glass is opaque 

 to the blue. A similar experiment would be to take a red 

 glass and a blue glass and let a beam of light pass through 

 both. As this beam of wfcite light passes through the blue 

 glass plate all the red and yellow would be absorbed and 

 nothing but the blue be permitted to pass. But blue light 

 is absorbed by the red pane, which allows only red light to 

 pass through, but as all the red light was absorbed by the 

 blue plate, no light at all passes through. The two trans- 

 parent glasses act like an opaque object. 



Second, what is the explanation of the color of the objects 

 around us, such as the cover of a book, the tint of a ribbon, 

 or the shade of a paint ? The explanation is similar to that 

 given in the case of transmitted light. White light falls on 

 the ribbon, and if this ribbon should reflect all the light 

 that it received, it too, would appear white. But the rib- 

 bon absorbs some of the colors and reflects only those colors 

 which it cannot absorb. In the case of the red ribbon, 



