Polytechnic Association. 913 



of green are absorbed and destroyed, or changed to other forms of 

 force, and only the green-appearing rays are reflected to onr eyes. 

 So again the red objects absorb all but the red rays. 



In proof of this I will now illuminate this brightly colored 

 banner with light of various tints in succession, and you will see 

 that in each single color only the object of the same color shows, all 

 the others looking black ; while if the colored objects could change 

 the color of the light, then certainly the red object would still look 

 red in the green light, and the green objects would show in the red 

 light. 



Such being the all but universal condition of objects in relation to 

 color, you will appreciate the surprise which was experienced by the 

 first observers when they encountered substances which, illuminated 

 with one kind of light or color, exhibited another. 



To give you an idea of such a phenomenon I have here arranged 

 two rough pictures of a flower with leaves and buds alike in outline, 

 but one, as you perceive, showing by the ordinary light many vivid 

 colors, while the other shows only a yellowish shade in parts, and that 

 so faint that it might easily escape notice. 



If now these are both illuminated with bright yellow light, the 

 highly colored picture fades and loses all its distinctive hues, as did 

 the banner before, and is as colorless as its companion, which remains 

 in this light unchanged, and so also if red or green lights are used; 

 but if violet light is substituted, see what a wonderful change we 

 have. The colored picture indeed is as dull and tintless as with 

 the yellow light, but the uncolored one now seems fairly to glow 

 and burn with brilliant green and blue and red. 



Here is certainly an astonishing thing ; a substance or substances 

 which, receiving violet light, neither absorb nor reflect it, but change 

 it into blue or green or red. 



In the first place, then, let us inquire more particularly as to the 

 limiting conditions of the exciting light. We have seen that in the 

 substance just used, red, green and yellow light will not produce the 

 effect, but that violet, or that a mixture of blue and violet, will. Is 

 this, then, universal ? Thousands of experiments have evoked answers 

 to this question, and I will give you in brief a summary of their 

 replies. 



Yiolet light alone develops fluorescence in all bodies that are capa- 

 ble of exhibiting this phenomenon, but every sort of color may evoke 

 fluorescence from some body. Thus, in the picture of a flower which 

 [Inst.] 58 



