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LX. Account of two Experiments on Accidental Colours; with 

 Observatiofis on their Theory. By Sir David Brewster, 

 LL.D., F.B.S. Sf V.P.R.S. Ed. 



ALTHOUGH considerable progress has been made in 

 observing the phenomena of accidental colours in all the 

 various forms under which they present themselves, yet we 

 know almost nothing of the manner in which light acts upon 

 the retina when it is rendered insensible to particular colours 

 of the spectrum. In making some experiments on this sub- 

 ject, I observed two very curious facts, which possess some 

 interest independent of their theoretical relations. 



1. It has been long known that when the eye is under the 

 influence of a luminous impression which causes it to see the 

 accidental or complementary colour of the exciting light, the 

 accidental colour often vanishes and re-appears. If a smart 

 blow is given to the head when the eye sees the accidental 

 colour in its first and brightest phase, it will instantly disap- 

 pear, so that the vibration thus suddenly communicated to 

 the retina restores to that membrane the sensibility to the 

 primitive or exciting colour which it had lost. I varied this 

 experiment by giving the blow to the head before the acci- 

 dental colour was seen, that is, when the eye was still fixed 

 upon the exciting colour. When this was done, and the eye 

 quickly turned upon the white ground, the accidental colour 

 was not visible. 



I was now anxious to learn whether the same effect would be 

 produced by strong vibrations communicated to the head 

 through the intermedium of the air, and with this view I had 

 a large gong powerfully struck close to my head when the 

 accidental colour was a maximum. I could not observe, how- 

 ever, the slightest change either in the intensity or duration of 

 the complementary impression. 



2. The influence of strong light in rendering the retina 

 partially insensible to red rays, even when these rays fall upon 

 a part of the membrane which has not been directly acted 

 upon by the strong light, has been finely shown by the ex- 

 periment of Dr. Smith of Fochabers*, and I have mentioned 

 in a former paper f that a stick of red sealing-wax may be 

 thus made to appear of a dark liver-brown colour. 



If we apply the strong light to the eye when the sensibility 

 of the retina has been locally diminished by looking at a red 

 object, a total insensibility to red light will be produced. 

 In order to observe this curious result in perfection, let the 

 eye be steadily fixed for some time upon a seal of red wax 



* Dr. Smith's paper will be found in Lond. and Edinb. Phil. Mag., vol. i. 

 p. 2.10. 1 //W.,p. 172. 



Third Series. Vol. 4-. "No. 28. May 1834. c 2 Z 



