57 
bers took part. It was closed by some remarks from Dr. Henry Fripp, of 
^hich we are enabled, by tbe kindness of that gentleman, to give a somewhat 
amplified statement. He said : — 
Lightness or darkness of hair, irrespective of the colour due to pigment, is 
the result of the optical properties of the structural elements concerned, and 
of the morphological arrangement of the tissues composing the hair or fur. 
Change from light to dark or dark to light may be adequately explained by 
considering the altered optical effects due to structural differentiation of the 
organic particles ; as for example, increased or diminished opacity, density, 
aggregation, fluidity, permeability by air, water, or liquid fats ; and certain 
changes of external and internal physical condition, such as dryness or 
moisture of the surface on which the light impinges, (hair being notoriously 
hygrometric) as also smoothness or roughness of the cell walls, the plane or 
angular form of the interior particles, &c. 
When we speak of white or black hair, our estimate of the relative white- 
ness is founded on the amount of light reflected from the hair viewed as a 
more or less opaque object, and not on the existence of any white pigment. 
Light falling on a hair is partly reflected from the surface, and partly 
transmitted to the interior, again to be reflected outwards, and to undergo 
a greater or less amount of dispersion. The specific properties of reflection, 
refraction, and absorption of light, possessed by the several constituent 
elements of hair structure, form important items in our estimate of the cause 
of change in appearance of the hair. The "lustre " of hair is not the same 
as that of glass or metal, (dyed hair has no lustre.) It is not a surface effect, 
but rather due to the various refractions of light and shadow going on in the 
organic matter or substance of the hair, and requires a certain thickness of 
substance as well as differentiation of the elements. Lustre also depends on 
the brilliancy of the positive colour (pigment), Reflection from the surface of 
a hair is affected by the nature of that surface, its smoothness or roughness — 
its flattened or cylindrical form — its depressed facets, or elevated ridges— its 
reticulated markings, (iridescence) &c. Thus a scaly or imbricate surface 
affects the reflection and transmission of light, according to the fineness, 
coarseness, irregularity, &c. of the surface marks. The epidermic scales of 
the cuticle are, for the most part however, sufficiently translucent to allow 
of a large amount of light passing to and from the interior, as pigment is not 
found collected in the cuticle. 
The further transmission of light into the interior depends, in the next 
place, on the nature of the surface of the cortical substance. The maximum 
reflection of light will take place when the outer surface of the cortex is most 
