914 Transactions of the American Institute. 



you first saw fluorescent, the material which gave a blue color is only 

 " excited " by violet light, that which gave a green color is excited by 

 even the lowest of the blue rays, and by all above, but with variations 

 in intensity ; that is, one color or particular length of wave acts upon 

 it more strongly than those of neighboring lengths. 



Lastly, the center of the flower was, you noticed, red, and the body 

 which developed this color is not only excited by the violet rays, but 

 also by those of red light, though not by all the colors between. 



So again, chlorophyl, which can be best obtained by exhausting tea 

 leaves repeatedly with boiling water, so as to remove all that water 

 will dissolve, and then treating them with strong hot alcohol, is 

 excited by certain particular wave of red, other special tints of orange, 

 some of green and special ones of violet. 



So we might cite a hundred cases, all varying more or less, but 

 yet all obeying this general law discovered by Prof. Stokes, that the 

 exciting light is always of a less wave-length than the fluorescent 

 light which it develops. 



Thus, violet light may develop a blue fluorescence, but a blue light 

 can never develop a violet fluoresence, and if, as in chlorophyl, the 

 exciting light is red, that developed must be of a lower red or longer 

 wave. 



Thus you see, when I expose this flask of chlorophyl to the beam of 

 united blue and red light now proceeding from the lantern, its original 

 olive green color is replaced by a magnificent luminous crimson, as of 

 blood. 



"We see then that sources of light rich in the violet rays will be the 

 most efficient in exciting this action of fluorescence, and of such 

 sources we have first of all in this regard the electric discharge 

 obtained from an induction coil. In proportion to the total amount 

 of tight it gives, this is the richest in very short waves ; that is, in 

 those visible as violet light, and those active in effecting chemical 

 change, though, as a rule, invisible. 



For such purposes as the present, this discharge, when taking place 

 in the little tubes containing highly rarefied pure nitrogen, is most 

 effective, and I have accordingly arranged here a multitude of these, 

 which we can illuminate with this immense induction coil. 



I turn them toward you and you perceive what a blaze of violet 

 light they emit, and now turning them away they light up the whole 

 stage, and cause every fluorescent object on it to glow. 



The electric discharge from a powerful galvanic battery, which we 

 have already used, and shall use again, is another efficient source of 



