^8 On the Colour of the Atmosphere and Deep Water, ^ 



boards are painted grey, one with a mixture of white lead and 

 charcoal ground with oil, the other in covering (Tun glacis de 

 charhwi the board previously prepared with white lead, in such 

 a manner that both boards have the same shade of light. Their 

 ^int will be found different ; the first will be bluish, and the 

 second grey without any mixture of blue. 



As transparent colouring substances lose in oil almost all the 

 coloured reflection which they possess in a pulverulent state, 

 and, therefore, approach to black in the mass, their mixture 

 with white produces opaline blue, by which the natural tint of 

 the colouring substance is modified. 



., All painters are aware of the striking difference which exists 

 between the colour formed by the mixture of the lac of cochineal 

 with white, and that which is produced by the same lac when 

 spread in a thin coating over a surface prepared with white ; 

 the first is violet, and the second has all the purity and bright- 

 ness of which this fine colour is susceptible. Artists, therefore, 

 who wish to obtain the most beautiful cochineal red in their 

 paintings, always use these lacs en glacis. Opaque and re- 

 flecting colouring substances, such as Naples-yellow, chromate 

 of lead, and yellow ochre, produce, like white lead, an opaline 

 blue by mixture with charcoal, and the effect is still more ob- 

 vious. These composite colours, according to the theory, ought 

 to produce only more or less obscure tints of yellow ; there re- 

 sult from them, however, decided tints of green, which are often 

 used to paint the herbage in landscapes ; in this case it is the 

 opaque colouring substance which is opaline. 



I have mentioned the most remarkable effects of the singular 

 property possessed by certain colouring substances, the mixture 

 of which produces opaline blue ; but there is a multitude of 

 less obvious modifications resulting from the mixture of compo- 

 site colours, which it is impossible to describe, but which may 

 be always foreseen by the following statement ; When xohite 

 fkad, or opaque reflecting colouring substances, are mixed with 

 hlaclCy (yr with transparent colouring substances, blue is pro- 

 dwed, and the primitive tint of the colouring substance is con- 

 seqitently modified. 



These modifications are often very slight, but they never 

 escape attentive observation. 



