Fluorescence and Photo- Chemistry. 765 



Variations in the intensity of the sunlight during the experi- 

 ment make no difference, as both cells are affected by them 

 to the same degree. 



After an exposure of three hours it was found that the 

 cell receiving the concentrated illumination showed a much 

 greater change than the one exposed to normal sunlight. 

 Its fluorescence was less, and the colour of the transmitted 

 light much redder (due to the formation of a greater quantity 

 of photo-eosine). 



The gain in the action by increasing the intensity is 

 probably even more marked than is shown by this experi- 

 ment. To perform the experiment in an ideal manner a 

 given amount of the solution should pass only once through 

 the concentrated light-beam. As a matter of fact, there is a 

 strong convection current upward, due to the heating of the 

 liquid at the focus, and the solution which has passed through 

 the beam and been partially decomposed mixes with the fresh 

 solution and dilutes it, so to speak. This will make the in- 

 creased action due to high intensity of the light less marked 

 than it would be if the solution could be kept at rest, and the 

 solar image moved over every portion of the cell in a per- 

 fectly uniform manner, so that each surface element would 

 receive equal exposure. 



This relation between the intensity of the light and the 

 rate of change is evidently extremely important, and it is 

 being subjected to a careful quantitative investigation at the 

 present time in collaboration with Mr. Subkow, one of my 

 students. 



Unstable dyes, which are non-fluorescent, do not behave 

 in this way, the bleaching in the two cells being equal in 

 amount. 



We now have at our disposal a mirror of 32 inches aper- 

 ture and 18 inches focus, with which platinum-foil can be 

 instantly melted by concentrated sunlight. With this 

 mirror the photo compounds can be made in quantity 

 sufficient for chemical examination. 



