64 
of which, rows of cells are arranged with a tolerably regular rectangular 
outline — the walls of the cells are partitions of horny substances of a soft 
consistence ; the substance of the walls of the outermost cells being con- 
tinuous with the inner layer of the cortex. In consequence of the absence 
of pigment, the dark shaded outline of this network of cells, which gives the 
appearance of a window mapped out by bars, is due to the refraction and 
opacity of the cell walls, or septa seen edgewise. 
The whole of this structure of the shaft of the hair being therefore nothing 
more, in an optical point of view, than a series of air-filled cells, with 
separating and enclosing cortical substance which is clear and homogeneous, 
the result is, almost total refraction oj light from the pith-like mass oj cell 
membrane— hut this light being broken up without separation into colour, a 
white tint like snow is produced— mc\\ as, produced optically, in the same 
way, characterizes the furry coat of these animals. Ermine fur is similar, 
and the varieties of coloured fur differ only in gloss and in the presence of 
different coloured pigments. 
The reflection of a white tint is illustrated by mica — when a plate of 
transparent mica is split into very thin laminae each is in itself transparent and 
colourless. If now the laminae are piled one over the other, a white tint 
results from the continuous reflection from the separate surfaces having a 
thin stratum of air interposed bebween them. 
But, in addition to this, a certain lustre is observed which is due to the 
repeated reflection, as well as refraction, going on in the alternate strata 
of air and mica. The hair has a lustre due to the horn substance, which 
lustre is of the kind peculiar to semi-transparent matter (vitreous lustre 
being the type, in contra- distinction to the metallic lustre due to opaque 
bodies). 
To simplify the question of colour, I have treated it as a quality of the 
pigment. The usual explanation of a coloured substance is, that all the 
colours of decomposed white light are absorbed by the ©olouring substance, 
except that which the substance itself transmits or reflects. But absorption 
does not mean annihilation ; the absorbed colours have escaped the eye, 
have become invisible, but the undulations have disappeared to the sense 
of vision only because they have been translated into some other correlation 
of movement. Again, many organic structures cause the sensation of 
colour when the structure itself shews only those physical conditions by 
which the number of undulations, corresponding to the colour seen, is 
produced. Thus in the iris no coloured pigment is found, and similarly the 
colour of a hair, as seen by reflected light, varies often considerably from 
that observed in the pigments diffused in its substance. 
