49 
out of place, we must make some allowance for the tendency 
shown by men at all times to compare one beautiful object 
with another beautiful object, or one terrific object with 
another terrific object, without regard to exact literal truth. 
Generally speaking, I think we may safely say that no 
terms denoting colour, wherever met with, are to be con- 
sidered strictly correct and appropriate unless they are 
referred to some fixed and known standard. The errors 
due to actual colour blindness need hardly, I think, be taken 
into consideration, since the hues which the colour blind 
are unable to distinguish lie so far apart as to make it 
difficult for any one with normal eyes to conceive the possi- 
bility of so great a defect. 
The brilliant tints exhibited by the decaying foliage of 
the trees in this neighbourhood in the course of the 
autumn, forming a chromatic display such as those living 
near manufacturing towns have few opportunities of wit- 
nessing:’, have led me to think a little on the cause of the 
formation of these colours and its possible connection with 
chlorophyll, on the chemistry of which I have lately been 
making some experiments. 
The colour of the leaves of plants is a phenomenon which 
is probably never quite stationary at any period of their 
development. When lying rolled up in the leaf bud they 
are like underground shoots and other parts of plants that 
have not been exposed to light, almost white ; nevertheless 
they already contain a colouring matter called etiolin, the 
alcoholic solution of which is yellow and shows absorption 
bands similar to those of chlorophyll. Whether this etiolin 
on the leaf unfolding passes over into chlorophyll and 
whether it continues to be formed during the further stages 
of development is not known. All we know is that when 
the leaf expands it immediately becomes green from the 
formation of chlorophyll. It is, however, evident that more 
