384 Ewart . — The Action of Cold and of 
connexion that a substance transparent to the eye may be 
opaque to particular rays, and that a colourless tissue may 
reflect or absorb a considerable portion of the light falling 
upon it. 
The effect produced upon the chloroplastids is not, how- 
ever, directly proportional to the total amount of light which 
they absorb, for it is the blue-violet rays which produce the 
greatest photo-chemical effect, whereas the chlorophyllous 
absorption in this region of the spectrum is not nearly as 
great as it is in the red. 
The following experiments illustrate these points more 
clearly, and give more precise data concerning the duration 
as well as the intensity of the exposure which chloroplastids 
can successfully withstand. Perfectly accurate data could 
be obtained only by experimenting with a constant source 
of illumination, the intensity of which could be varied at 
will, for when sunlight is used its intensity may continually 
alter during the exposure. Hence in the following experi- 
ments the sunlight was concentrated and the exposure 
correspondingly shortened, while by using single cells or 
unilamellar cell-aggregates the intensity of light which 
reaches the chloroplastids is but little diminished. End-cells 
of Chara foetida and the laminar portions of the leaves of 
Elodea were therefore exposed in water to the sun’s rays, 
concentrated by means of an ordinary concave mirror, and 
previously cooled by passing through a cold solution of alum. 
The image of the sun thrown at the level of the microscope 
stage was rather less than g^th of the area of the reflecting 
mirror, but the photo-chemical intensity (for silver salts) of 
the concentrated sunlight was much less than it should 
theoretically have been, namely 25 to 30 times greater 
than that of the direct sunlight, or after passing through an 
alum solution only 8 to 10 times greater. The intensity of 
the light employed is indicated by the terms 4 S, 6 S, 8 S, &c., 
where S represents the full intensity of the strongest sunlight. 
Elodea canadensis , leaf-cells exposed in water, cooled by evaporation 
and with plenty of oxygen. 
