762 Prof. R. W. Wood on 



the eye and prism in suitable positions to bring the two 

 images together with a sharp line of separation between 

 them. This line was found to be practically invisible, 

 showing equality of the intensities and proving that the 

 law holds up to the very high intensities used. The lens 

 was then stopped down to 1/600,000 of its full aperture by 

 means of a sheet of black paper with a small pin-hole in it. 

 Even with this enormous reduction in the fluorescent spot on 

 the bottle c the fluorescence was distinctly visible, which 

 shows us that we can increase the intensity of a beam of 

 light, which is sufficiently intense to cause the decomposition 

 of the fluorescein molecules, six hundred thousandfold without 

 causing the instantaneous destruction of all of them — in fact, 

 an exposure of ten or fifteen minutes to the intense beam is 

 necessary before the transformation is complete. 



Perrhr's theory is that the intensity of the fluorescent 

 light emitted by a single molecule is independent of the 

 intensity of the exciting light, and that the increase in 

 the intensity of the total emitted light is due merely to the 

 circumstance that more molecules are being destroyed (or 

 transformed) per unit of time in the case of intense illumi- 

 nation. In support of this, we have the circumstance that 

 sooner or later the fluorescent solutions are bleached by 

 the action of the light. But this is also true of a large 

 number of non-fluorescent solutions of organic colouring- 

 matters. 



We can make the alternative hypothesis that all of the 

 molecules in the solution fluoresce to an equal degree, and that 

 as the intensity of the exciting light increases the amplitude 

 of the fluorescent radiation emitted by each molecule in- 

 creases to a corresponding degree. While this is going on 

 the solution is gradually undergoing decomposition by the 

 action of the light, as in the case of a non-fluorescent solu- 

 tion. It seems extremely desirable to devise experiments 

 which will enable us to choose the more probable of the two 

 hypotheses. Two such experiments suggested themselves. 



Are the Molecules Fluorescent only at the Moment of 

 Breakdown? 



It would appear as if some light might be throwm on the 

 two alternative hypotheses mentioned above if we can pre- 

 vent fluorescence and still allow light to act on the medium. 

 There are two w T ays in which this can be done. As has been 

 already stated, the fluorescence of an aqueous solution of 



