472 Sir David Brewster on the Colours of Natural Bodies. 
it absorbed only the red rays, the opalescent beam would have 
been red throughout the whole of its path: but as the different 
colours are absorbed in different proportions, and, in the pre- 
sent case, in the order of their refrangibility, excepting the 
blue and violet, the colour of the intromitted beam must vary 
from red to greenish-yellow, as these colours are successively 
taken out of it. 
The analysis of this experiment is very interesting, but as 
this is not the place to pursue it, I shall only remark, that I 
have observed the same phenomenon in various other fluids 
of different colours, that it occurs almost always in vegetable 
solutions, and almost never in chemical ones, or in coloured 
glasses; and that it is a phenomenon of opalescence or im- 
perfect transparency. One of the finest examples of it which 
I have met with may be seen by transmitting a strong pencil 
of solar light through certain cubes of bluish fluor-spar. The 
brilliant blue colour of the intromitted pencil is singularly 
beautiful. 
According to the Newtonian theory of colours, the green 
of plants is of the same order as the yellow and orange into 
which it is changed when it withers, in consequence of an in- 
creased density, or an enlargement of size in the tingeing cor- 
puscles. In order to put this opinion to the test of experi- 
ment, I extracted the yellow juice from the brilliant yellow 
leaves of the common laurel. This fluid becomes of a deep 
red at great thicknesses. It attacks the spectrum powerfully 
towards the extremity of the green space, a place where it is 
not touched by the green fluid. It then absorbs the yellow 
and violet, leaving a bright green, and converting the blue into 
violet. At greater thicknesses, the violet disappears, and the 
absorption advances gradually to the red. 
For the purpose of varying the experiment, I extracted the 
juice of the leaves of the privet, which become of a deep 
black violet when they wither, a colour which has not the 
most remote resemblance to any periodical tint. The fluid 
was a deep red colour, much deeper than that of the darkest 
port wine. It attacked the red part of the spectrum near the 
line B of Fraunhofer, at the same place that the green juice 
attacked it, leaving (wo red bands, the innermost of which 
vanished at an increased thickness. It then absorbed the 
violet and blue spaces generally; and having obliterated the 
middle of the green space, the absorption advanced to the 
orange rays at D. 
Now, in both these experiments, the action of the colouring 
matter of the decayed leaves is decidedly different from that 
of the green juice, and there is no appearance whatever of the 
