74 Color of the Air and of Deep Waters. 
Thus the difference which may exist in the size of the blood ves- 
sels and in their proximity to the surface is sufficient to produce all 
the shades of blue, violet, red and purple which are seen in the hu- 
man face, by the mixture of the opaline blue of the skin, with the 
red of the blood. 
The red color of the blood is not the cause of the blue tinge of 
the veins ; it might be black or green without occasioning any change; 
it is enough that the coloring principle absorbs all the light transmitted 
by the skin. ‘This result may be artificially produced by a very thin 
plate of ivory which has nearly the same effect as the skin. If a 
few drops of ivory black, prussian blue, cochineal, or bile, sufficiently 
dense to be opake, be placed on one of its surfaces, they produce 
alike a blue tint on the opposite surface because they equally absorb 
all the light transmitted by the ivory. But if instead of a coloring 
matter which absorbs light, we use an opake reflecting coloring sub- 
stance, we have a tint compounded of opaline blue and that of the 
color employed. 
The red oxide of lead placed on the ivory, gives on the opposite 
surface, aslight tinge of carmine. Some painters avail themselves of 
this property of ivory, in sketching the cheeks and lips of their por- 
traits by placing a coat of minium on the opposite surface, and thus 
obtain indirectly the effect of a slight use of carmine. | 
But if instead of minium, Naples yellow be put on, there is on 
the opposite surface a green spot. In both these cases then, the 
opaline blue is mingled with the proper tint of the opake reflecting 
color, while the blue alone appears when the applied color absorbs 
the light transmitted by the ivory. 
The mixing of colors in oil painting furnishes still more evident- 
ly an opaline blue. ‘The most common case is the mixture of white 
with vegetable black which produces a bluish shade. Various wri- 
ters have adverted to this, and as indigo and prussian blue, in mass, 
approximate to black, it was thought in former days, that blue was a 
‘mixture of light and shade ; but the blue produced on this occasion, 
belongs exclusively to white and not to black as is proved by the 
following process: two plates are painted of a grey color, one by a 
mixture of ceruse and charcoal ground in oil, the other by super- 
adding to a coat of white a glazing of charcoal, so that they may 
both have the same depth of shade; the first will be bluish, the sec- 
ond grey without a mixture of blue. 
