ACCIDENTAL COLOURS. 



261 



be of a bluish-green, in a circular spot of the same dimensions as the 

 wafer. This bluish-green image is called an ocular spectrum, because 

 it is impressed upon the eye, and may be retained for a short time; 

 and the colour bluish- green is said to be the accidental colour of the 

 red. If this experiment be made with wafers of different colours, other 

 accidental colours will be observed, varying with the colour of the 

 wafer employed, as in the following table : 



Colour of the 

 Wafer. 



Red, 



Orange, 

 Yellow, 

 Green, 

 Blue, 

 Indigo, 

 Violet, 

 Slack, 

 White, - 



Accidental Colour, or Colour of the 

 Ocular Spectrum. 



- Bluish-green. 



- Blue. 



- Indigo. 



- Violet, with a little red. 



- Orange-red. 



- Orange-yellow. 



- Yellow-green. 



- White. 



- Black. 



Black. 



Accidental Colours. 



If all the colours of the spectrum be ranged in a circle, in the pro- 

 portions they hold in the spectrum itself, as in Fig. 118, the acci- 

 dental colour of any particular 

 colour will be found directly oppo- 

 site. Hence the two have been 

 termed opposite colours. 



It will follow, from what has 

 been said, that if the primary 

 colour, or that to which the eye 

 has been first directed, be added 

 to the accidental colour, the result 

 must be the same impression as 

 that produced by the union of all 

 the rays of the spectrum of white 

 light. The accidental colour, in 

 other words, is what the primitive 

 colour requires to make it white 



light. The primitive and accidental colours are, therefore, comple- 

 ments of each other ; and hence accidental colours have been called 

 complementary colours. They have likewise been termed harmonic, 

 because the primitive and its accidental colour harmonize with each 

 other in painting. It has been supposed, that the formation of these 

 ocular spectra has frequently given rise to a belief in supernatural 

 appearances, the retina, in certain diseased states of the nervous sys- 

 tem, being more than usually disposed to retain the impressions, so 

 that the spectrum remains visible for a long time after the cause has 

 been removed. Such appears to be the view of Drs. Ferriar, 1 Hib- 

 bert, 2 and Alderson, 3 the chief writers in modern times, on appari- 

 tions. This subject may be the theme of future discussion. It will be 

 sufficient, at present, to remark, that the great seat and origin of 



1 An Essay towards a Theory of Apparitions, Lond., 1813. 



2 Sketches of the Philosophy of Apparitions, Edinb., 1825. 



3 An Essay on Apparitions, &c., Lond., 1823. 



