Carbon Assimilation. 
43 
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^/x and 
524/z/x. 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 1 
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, 2 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. 
1 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). 
