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None of the processes above mentioned are comparable with changes of 
colour in the particles of a circulating fluid ; e. g. in the petals of a flower. 
The colouring matter of flowers, is not in the cellulose wall of the plant- 
cell, but in the fluid contents. Such colour may come and go rapidly, be 
intense or delicate, or disappear in an equally short time, but there is no 
question of white or black here, or of altered reflection or refraction ; it is 
simply of gradations of colour, whether single or \ mixed tints. 
Respecting the white furs and hair of animals, the microscope yields us a 
full explanation of the facts. The difference between colourless and coloured 
hair is one of pigment or its absence. The appearance of whiteness or dark- 
ness of an object is an optical phenomenon, explicable by material conditions 
such as are exhibited by the elegant hair and wool structures of mammals 
when placed under a microscope. In viewing such textures by transmitted 
light, solid parts which strongly reflect or refract light, will be seen as more 
or less opaque objects ; fluids which are colourless, and transmit light, will 
be scarcely noticeable; and coloured fluids absorbing all portions of the 
spectrum but those corresponding to their own colour, reflect and transmit 
their colour equally whether viewed by reflected or transmitted light. But 
a colourless part is not necessarily transparent, as it may appear with a 
white tint, which has more or less "body," giving the idea of substance. 
Thus if we examine a dark coloured hair by transmitted light, it will appear 
black in proportion to the density and closeness of the pigment — being 
opaque, it obstructs the passage of light. The same hair seen in reflected 
light, will exhibit its own proper colour if the cuticle under which the pig- 
mented cortex lies is transparent ; but the surface markings of the cuticle 
will be distinguishable as fine lines projected over the subjacent pigment. 
Again, if the pigment disappear and be replaced by fatty molecules, the 
hair will appear, by reflected light, of a dull white colour, with a consider- 
able body, and by transmitted light, instead of being transparent, it will 
appear as opaque, and therefore as dark as the pigmented hair, care being- 
taken to exclude all reflected light. 
If, again, a large sized central core of air-filled cells exist, the hair will 
shew white by reflected and dark by transmitted light, though there be 
little or no colour. Lastly, in a hair having considerable thickness, and 
containing tissues which at different planes of its thickness are composed 
of transparent horny matter, molecular pigment, fatty granules aud air-filled 
cells, the appearances by reflected light will be due to an infinite variety of 
internal refractions and reflections, and will shew various shades of dark 
when the substance absorbs, or partially transmits light ; but various white- 
ness of tint where the substance is opaque. And these appearances will be 
reversed when the hair is viewed by transmitted light, if the hair be 
partially transparent. 
