740 GENERAL CONDITIONS OF PLANT-LIFE. 
the action of light on plants varies with its intensity, as that of temperature with its 
elevation, does not admit of a doubt, and is obvious in all physiological observations. 
There can scarcely be said, however, to be any exact investigations on this point ; 
and the great obstacle to their accomplishment is that we have at present no method 
of measuring the intensity of rays of light of any particular refrangibility in terms of 
a fixed unit which can be applied to plants. As far as concerns the highly refran- 
gible rays, e. those which have the greatest mechanical effect, we are compelled to 
adopt the photo-chemical method of Bunsen and Roscoe\ which however gives no 
information respecting the different intensity of the red, orange, and yellow light, 
and can only be applied with great difficulty to experiments on vegetation. In the 
photometry of the less refrangible rays, on the contrary, we must always have 
recourse, according to the ordinary method, to the sensitiveness of the eye, i. e. to 
brightness, which cannot be considered in itself to be an actual objective measure of 
the intensity of the light, though it may be assumed under certain circumstances 
that increase or diminution of subjective brightness corresponds to increase or 
diminution of objective intensity. In describing the relation between the intensity 
of light and vegetation, we have therefore at present, with a few exceptions, to 
employ the ordinary expressions dark, dull, bright, dazzlingly bright, &c., and to 
assume that they correspond to certain objective intensities. There is one case in 
which this relation between the subjective sensitiveness of the eye and the action 
upon vegetation of the light which causes it can be very strikingly proved ; Pfeffer 
has shown that the curve of the subjective sensitiveness of the eye for the colours of 
the solar spectrum coincides exactly with the curve expressing the power of different 
regions of the spectrum in decomposing carbon dioxide ^. This coincidence must 
however at present be considered purely accidental^, and cannot be extended to 
other phenomena. If the sunlight or diffused daylight which reaches the observer 
were always of the same intensity, it would be easy to regulate artificially, according 
to definite gradations, the intensity of the Hght that acts on the plant. But since the 
light of incandescent bodies (such as the Drummond's light^) contains the same 
rays as sunlight and acts similarly on the functions of plants, constant sources 
of light of a definite intensity can in this way be arranged, which will admit of 
gradual adjustment, in order to study the influence on vegetation of light of different 
intensities. 
If we now turn to the observations on record, those of Wolkoff are the only 
ones in which actual measurements have been made. With the assistance of 
the photometric method contrived by Bunsen and Roscoe^ he showed first of 
all that changes in the intensity of the highly refrangible light do not stand in 
any appreciable relation to the exhalation of gas by water-plants. This is an 
intensity of light and its brightness to the eye, see the paper quoted above and the literature there 
referred to. 
^ See the admirable paper by Wolkoff in the Jahrb. für wiss. Bot. vol. V, p. i. 
^ Pfeffer in Sitzungsber. der Ges. zur Beförderung der ges. Naturv^dss. für Marburg, 1872, 
May 16. 
^ See note on p. 747. 
* See Herve Mangon, Comp, rend, 1861, p. 243. — Prillieux, ibid. 1869, p. 408. 
^ Bunsen and Roscoe, Pogg. Ann. vol. 108. 
