On the Colour of the Atmosphere and Deep Water, 57 



their proximity to the epidermis, are sufficient to produce all 

 the tints of blue, violet, red, and purple, observed on the hu- 

 man countenance, by means of the mixture of the opaline blue 

 of the skin with the red of the blood. 



v;lThe red colour of the blood is not the cause of the blue tint 

 of the veins ; it might be black or green, without producing any 

 change, it being sufficient that the colouring substance absorb 

 all the light transmitted by the skin. This result may be olv 

 tained artificially by means of a very thin plate of ivory, which 

 produces nearly the same effects as the skin. If one of its sur- 

 faces be spotted with ivory black, Prussian blue, cochineal, vert de 

 vessie, prepared with much gum, and so thick as to be no longer 

 transparent, these colouring substances will equally produce a 

 blue tint on the opposite side, because they absorb all the light 

 transmitted by the ivory. But if, instead of a colouring mat- 

 ter which absorbs the light, the experiment be performed with 

 an opaque reflecting substance, the resulting tint is composed 

 of opaline blue and the colour of the subsmnce employed. The 

 following are some examples of this. 



The red oxide of lead (minium of painters), placed on an 

 ivory plate, produces on the opposite surface a slight tint of car- 

 njine. Some painters avail themselves of this property of ivory 

 in sketching the cheeks and lips of their miniatures, by applying 

 a layer of minium on the corresponding parts of the opposite 

 surface ; an artifice by which the effect is more easily obtained; 

 than by the direct application of a slight tint of carmine. But 

 if Naples-yellow be used instead of minium, the opposite surface 

 is greenish. We observe in these two examples that the opa- 

 line blue is mingled with the proper tint of the opaque reflect- 

 ing body, while the blue alone appears when the colouring sub. 

 stance absorbs the light transmitted by the ivory. 



The mixture of colours in oil painting, shews the production 

 of opaline blue in a still more evident manner. The best known 

 example is the mixture of white with vegetable charcoal, which 

 produces a bluish tint. As indigo and Prussian blue in the 

 mass approach to black, some have thought that the blue is a 

 mixture of light and shade ; but that the blue produced by the 

 mixture alluded to above, is owing exclusively to the white and 

 not to the black, is proved by the following process : two small 



