250 THE EYE. 



forget that what we mean by this term is, not that the 

 rays are yellow in themselves, but only that they have the 

 power of producing that sensation in us when they enter 

 our eyes undergo, in the case of the buttercup and the 

 chin, two rejections : one from the petals of the flower, by 

 which they are separated from the other rays, and the oth- 

 er from the chin. Any color may be thus reflected a sec- 

 ond time, and the effect is more or less distinct accord- 

 ing as the second surface is more or less shaded from all 

 extraneous light that is, light coming to it from other 

 sources. Thus almost any object held near a red curtain 

 which the sun shines upon will look red by reflected light. 

 In this case the red light from the curtain constitutes so 

 large a portion of all the light which the object receives 

 that the reflection of it becomes visible. 



In the same manner, all the objects in an ordinary room 

 reflect each its own colored rays to every part of the room. 

 These rays mingle and blend with each other in passing 

 through the air, and if we hold up a sheet of paper as a 

 screen, they all fall upon it together, in combination with 

 the white light of the sun, so that the paper reflects only 

 a mingled and general light to our eyes. But if a lens is 

 interposed in a proper manner between the paper and any 

 group of these objects, and all other light is excluded, then 

 the differently colored rays from all these objects, and from 

 the different parts of the same object, are made to converge, 

 and are brought to a focus, each in its proper place, and a 

 distinct image of the whole group is formed, with all the 

 parts in their proper position, and of their proper color. 



How this is effected is shown very clearly in the image 

 of the lily in the engraving. The rays from only two 

 points (A and B) are shown, but the same effect takes 

 place in respect to the light issuing from every other point 

 in the flower. All these rays, in passing through the air, 



