246 THE ORIGIN OF LIFE 



light itself that was measured, and the objection 

 does not apply. 



M. Camichael uses a thick screen of uranium glass 

 7 cm. in thickness to cut out the more refrang- 

 ible rays, and in so doing the light from the lamp 

 corresponding to the fluorescent periods is con- 

 siderably cut down, so that that transmitted 

 by the screen would undergo much variation of 

 absorption in passing through another such piece 

 of uranium glass, as the transmitted light has already 

 been filtered of the particular rays whose absorp- 

 tion it is proposed to measure, and a negative 

 result is therefore to be expected. But this nega- 

 tive result indirectly confirms the fact that the 

 fluorescent spectrum is probably made up of fine 

 lines or bands, as the effect of sifting or filtering 

 of rays would thus still become more marked. It 

 is diflicult to judge by merely looking at the 

 transmitted spectrum what actually takes place. 

 The phenomenon of the change of absorption pro- 

 duced by fluorescence has since been investigated more 

 fully from a spectroscopic point of view by Nichols 

 and Merritt.-^ The difiiculty which I encountered 

 when studying this subject was the uncertainty of 

 photometric measurements, and my experiments were 

 primarily directed to aff'ording a satisfactory proof 

 of the existence of the phenomenon, and I did 

 not regard my results as conclusive till they 

 were tested by totally distinct and independent 

 methods as well as photographed. Nichols and 



1 he. cit. 



