70 Conjectures respecting the Principles 



In attempts to call up colours in this way, many 

 precautions are necessary, to which the most scrupu- 

 lous attention must be paid, otherwise the experiments 

 will not succeed. Care must be taken to exclude all 

 coloured light in illuminating the slips of paper; and, in 

 preparing that slip which is designed for exhibiting the 

 imaginary colour, the quantities of black and of white 

 powder that are mixed must be so adjusted to each 

 other that, when the surface of the slip is covered with 

 it, the quantity of light reflected from it to the spec- 

 tator's eye must be precisely equal to that reflected 

 from the surface of the other coloured slip, for this 

 equality is essential to the purity and brilliancy of the 

 colour called up. But this equality can only be found 

 by actual trials with several slips of deeper and lighter 

 shades. That slip which takes the clearest and bright- 

 est colour is to be chosen. 



When experiments of this kind are attempted to be 

 made with oil colours, other and still greater difficul- 

 ties will occur ; for the oil used for fixing the colours 

 diminishes in so great a degree both the brilliancy 

 and the purity of the light reflected from the surfaces 

 of coloured pigments that the light reflected from an 

 oil painting cannot be expected to produce the same 

 brilliant appearances which are exhibited by the mix- 

 tures and contrasts of the uncontaminated and brilliant 

 colours of pure light. 



But although it may be impossible for painters, with 

 their imperfect colours, to produce effects that will bear 

 a close comparison with those magic appearances of 

 which we have been giving an account, yet there can 

 be no doubt but that the knowledge of those facts, and 

 of the theory by which they are explained, may be very 

 useful to them. 



