150 Carbon Assimilation. 



As a green filter was used a solution obtained by mixing a 

 solution of potassium monochromate with ammonical copper oxide. 

 This solution let through light of wave lengths between 512/i/i and 

 524fjt.jx. No quantitative data were obtained in regard to the 

 coefficients of transmission. From those coefficients of transmission 

 measured and from the curve of distribution of energy in the spectrum 

 of the source of light it is possible to construct curves showing the 

 distribution of energy in the various regions of the spectra of the 

 light let through the filters. So, for instance, the distribution of 

 energy in the solar spectrum has been examined by Langley (1882) 

 and Fig. 18 shows the distribution of energy in the solar spectrum 

 and the distribution of energy in the light let through Kniep and 

 Minder's red and blue filters. 



Kniep and Minder only performed their experiments on 

 cloudless days at Naples between 11 a.m. and 2.30 p.m. when the 

 intensity of the light and the distribution of energy in the spectrum' 

 remained moderately constant. Heat rays were excluded by the 

 use of screens of distilled water. 



It is to be regretted that these authors, after having realised 

 the essential facts of energy distribution in the spectrum and after 

 introducing reliable methods, should render their experiments 

 ineffective by using a method for measuring carbon assimilation 

 which is one of the most unreliable. This method, which consists in 

 measuring the rate at which bubbles of gas are given off by an 

 assimilating submerged water plant,^ has recently formed the subject 

 of an investigation by Kniep himself (1915), who shows how many 

 and serious are the sources of error in it. In order to employ this 

 method, Kniep and Minder had to reduce the intensity of radiation 

 by a series of screens of different substances: water, copper 

 sulphate, potassium dichromate, more screens being used for the red 

 than the blue in order to bring the intensity of the radiation to the 

 same value in the two cases. By doing this, of course they 

 necessarily alter the distribution of energy, and the values obtained 

 for transmission and distribution of energy to which we have already 

 referred, have not much bearing on the actual experiments. 



' The relative intensity of the light of the blue part of the spectrum is very 

 small in the morning, increases towards mid-day, and falls off again in the 

 evening. 



^ This method, generally known as the " bubbling method," was due, like 

 so much in plant physiology, to Sachs. Accounts of researches in which it was 

 used are to be found in the works of, e.g., Pfeffer (1897), Reinke (1883, 1884), 

 Pantanelli (1903) and Treboux (1903). 



