

88 Additional Notes. 



Thus if you make a dot with ink in the centre of a circle of red silk 



*/ 



the size of a letter-wafer, and place it on a sheet of white paper, and 

 look on it for a minute without moving your eyes; and then gently 

 turn them on the white paper in its vicinity, or gently close them, and 

 hold one hand an inch or two before them, to prevent too much light 

 from passing through the eyelids, a circular spot of pale green will 

 be seen on the white paper, or in the closed eye ; which is called the 

 ocular spectrum ^of the red silk, and is formed as Dr. Darwin shows 

 by the pandiculation or stretching of the fine fibrils, which constitute 

 the extremities of the optic nerve, in a direction contrary to that, in 

 which they have been excited by previously looking at a luminous 

 object, till they become fatigued ; like the yawning or stretching of 

 the larger muscles after acting long in one direction. 



If at this time the eye, fatigued by looking long at the centre of 

 the red silk, be turned on paper previously coloured with pale green; 

 the circular spot or ocular spectrum will appear of a much darker 

 green; as now the irritation from the pale green paper coincides with 

 the pale green spectrum remaining in the eye, and thus excites those 

 fibres of the retina into stronger action; on this account some colours 

 are seen more distinctly, and consequently more agreeably after others ; 

 or when placed in the vicinity of others; thus if orange-coloured 

 letters are painted on a blue ground, they may be read at as great 

 distance as black on white, perhaps at a greater. 



The colours, which are thus more distinct when seen in succession 

 are called opposite colours by Sir Isaac Newton in his optics, Book I. 

 Part 2, and may be easily discovered by any one, by the method 

 above described ; that is by laying a coloured circle of paper or silk 

 on a sheet of white paper, and inspecting it some time with steady 

 eyes, and then either gently closing them, or removing them on 

 another part of the white paper, and the ocular spectrum or opposite 

 colour becomes visible in the eye. 



Sir Isaac Newton has observed, that the breadths of the seven pri- 

 mary colours in the sun's image refracted by a prism, are propor- 

 tioned to the seven musical notes of the gamut, or to the intervals of 

 the eight sounds contained in an octave. 



From this curious coincidence, it has been proposed to produce a 



