

1905. | On Vegetable Assimilation and Respiration. 435 
This failure to show a marked increase with marked increase of light proves 
that the assimilation is limited by the temperature of 32° C. For the last 
reading the light is diminished again, while the temperature is maintained 
and there is no serious fall in the assimilation. 
The assimilation values are then throughout limited by the temperature in 
the same manner as with normal leaves, and we have in one experiment the 
two instructive cases of (1) increase of light with stationary (limiting) 
temperature not causing increase of assimilation, and (2) increase of tempera- 
ture with stationary (non-limiting) light causing increase of assimilation. 
Section V.—TZhe Photosynthetic Value of Full Insolation. 
In this section we propose to give further precision to our knowledge of the 
relation of temperature and illumination to assimilation. We propose to 
ascertain what amounts of illumination correspond to certain assimilation 
maxima, in other words to measure the amount of assimilation corresponding 
to definite intensities of insolation. 
The procedure consists in experimenting with exactly known fractions of 
full midday sunlight. 
Various methods have been used for reducing the intensity of sunlight to a 
known extent. We made a few experiments with a method based on varying 
obliquity of incidence of the light and also with one employing different 
positions in the diverging cone of light from a condenser-lens. These could 
not be accurately applied without elaborate adaptation. The “photometer- 
wheel ”* of rotating sectors can be used to transmit any desired fraction of 
total incident light, but seems inadmissible for such work as ours because it 
fractions the light a tame and not in intensity. If set to half opacity it 
allows the full intensity of light to pass for half the ‘time, and when rapidly 
rotating this gives to the hwman eye an impression of steady light of half 
intensity. For an assimilating leaf it would give only illusory results except 
when the light was a “limiting factor.” Thus if, in a particular case, a leaf 
were limited by its temperature to using just 50 per cent. of the total 
sunlight, then halving the actual intensity should not diminish the assimilation 
(provided of course that the temperature were kept up), while halving this 
total illumination in time should reduce the assimilation to one-half.t 
Finally, we made use of the ideally simple method of employing fractioning 
screens made of thin metal plates perforated with holes.j These were placed 
across the mouth of the 4-foot tube directed towards the sun, that had been 
* See Abney, ‘ Phil. Trans.,’ 1887, and Langley, ‘ Phil. Mag.,’ 1889, vol. 27. 
t See, however, Note B, p. 459. 
+ Oar attention was directed to these by Professor Liveing. 
Zee 
