248 Notes 



a small quantity of white light, and our business is to explain 

 why it should appear green to the eye. 



13. Keep in mind that the idea of a perfect shadow excludes 

 all light, and that the space D v is an imperfect shadow, illum- 

 ined, as we have seen, with a small ponion of white light. 

 Let this small portion of white light be considered as made up 

 of red light and green light, according to what has been stated 

 above, in section 1 2, and the reason of the phenomenon will 

 be readily understood. For we must now attend to the strong 

 red light which passes through the glass c, and covers the paper 

 everywhere, except in the space d v, where it is intercepted : 

 the effect of this strong light coming up to the very boundaries 

 of the shadow d v is such as to incapacitate the eye from 

 seeing at the same time the weaker red light contained in the 

 shadow D V, which we have proved to be really of a weak dull 

 white colour, but which, because its red light cannot be seen, 

 appears green to the eye. 



14. This effect of rendering the organs of perception insen- 

 sible to weaker excitations, by strongly exciting those organs, 

 is analogous to the constitution of the human frame in many 

 instances. Accustom the eye either to much light or to intense 

 colours, and, for a time, it will hardly discern anything by a 

 dull light or by feeble colours, provided the feeble colours be 

 of the same kind with the previous strong ones. Thus, after 

 it has been excited by an intense red, for example, it will, for 

 a time, be insensible to weak red colours, yet it will still easily 

 perceive a weak green or blue, etc., as in the instance before us 

 respecting the shadovv D v, where the green part of the com- 

 pound still affects the eye, after the red has ceased to produce 

 any effect, owing to the previous excitation of a stronger red.' 



' This distinction should always be kept in mind, for, unless the eye has been abso- 

 lutely injured or weakened by excessive excitation, there is reason to believe that strong 

 excitations of it, whether immediately preceding'weaker ones, or contemporaneous with 

 them, much impro've its sensibility in regard to those weaker ones, provided only that 

 they be of a different class. If the eye has been excited by a lively red colour, it will 

 scarcely perceive a weak red, but it will perceive a weak green much better, on account 

 of the previous excitation by the strong red ; and the reason may be that, in looking at 

 a red colour, the eye wastes none of that nervous sensibility which is necessary for its 



