190:1] The Cosmical Function of the Green Plant 4i>7 



of radiation, was formulated by me, and I may add that I could not 

 ;affirm the second half of the truth for the simple reason that it had 

 already been affirmed before me, and consequently before Lommel, by 

 Jamin and Edmond Becquerel. It is only a matter for wonder that, 

 as a physicist, Lommel was not acquainted with Jamin's well-known 

 text-book and the perhaps still better known book of Becquerel " La 

 Lnmiere, ses Causes et ses Effets." 



But of course the main thing was not to express an idea, however 

 -exact, but to furnish an experimental proof of its exactness. Nullius 

 in rerba is now, as it was centuries ago, the watchword of every man 

 of science. In other words, it was necessary to repeat Draper's 

 celebrated experiment without falling into his error, and there was 

 the chief difficulty. It is generally admitted that this proof was 

 furnished for the first time by my old friend Professor N. M tiller, but 

 I am sorry to say that on this point I am again obliged to contradict 

 the general opinion. In fact M tiller never furnished the direct proof 

 of this connection between the chemical process and the absorption 

 of light, and he could not do so because he did not possess the only 

 means of escaping Draper's error. In his first work, previous to my 

 researches, he experimented in a tolerably pure spectrum, but the 

 light intensity was then insufficient and he could not actually obtain 

 any reduction of the carbon dioxide, but only inferred that it took 

 place, judging by the differences in the intensity of the respiration. 

 In his second paper, which appeared after my publication, he could 

 -affirm directly the reduction of carbon dioxide, but only at the cost 

 of the purity of his spectrum. In fact the width of his slit was nearly 

 the same as in Draper's experiment, and the result was practically the 

 same ; this time he obtained the maximum effect in the yellow rays. 



If I allow myself to insist on this point somewhat in detail it is not 

 from any vain desire to establish my priority, but because it is my 

 firm conviction that the method adopted by me is now, as it was at 

 that time, the only means of avoiding Draper's error. Mtiller did not 

 devise any means of avoiding and consequently had no chance of 

 escaping it. The dilemma, as fully exemplified by Mtiller's failure, 

 was the following : if the spectrum is pure the intensity of light is 

 not sufficient to obtain a reduction of carbon dioxide; if, on the 

 contrary, we open the slit in order to increase the intensity of the 

 spectrum, we may be sure beforehand of obtaining the maximum 

 effect in its middle, somewhere in the yellow or green rays. There 

 was but one means of escaping the two horns of this dilemma it 

 was to increase the intensity by diminishing the dimensions of the 

 spectrum ; but then the leaf surfaces being reduced in proportion, the 

 quantities of gas to be analysed would be too small to be measured in 

 such gasometric apparatus as the chemist of the time could put at the 

 disposal of the botanist. 



