1835.] Composition of White Light 117 



part of it. The motion of the eye, which when the object 

 is placed upon a white ground, produces the complemen- 

 tary colour upon the margin, causes the edge of the object 

 itself to re-assume its usual appearance, and to become even 

 more vivid than when the eye is first directed to it ; both 

 the primary and the accidental colour being exalted by an 

 alternate motion of the eye from one to the other, as we 

 have remarked in a former experiment. 



The production of accidental colours may be accounted 

 for on principles which are not confined to the sight of the 

 eye, but which extend to all the other senses. The first 

 sensation, whether of sight, of taste, of smell, of feeling, or 

 of hearing, is always the most vivid ; and the impression 

 gradually declines from its commencement, so that, if long 

 continued, without intermission,* we at length become 

 unconscious of its existence. 



Another principle, common to all our senses, is, that 

 when the organs are under the influence of strong impres- 

 sions, they are rendered incapable of being acted upon by 

 weaker impressions of the same kind, at least with the usual 

 effect.. 



Upon the latter principle, if we go from the open day 

 light into a room in which there is very little light, upon 

 our first entrance we cannot even perceive the objects which 

 surround us, in a short time, however, we discover their 

 outlines, which are gradually filled up, until at length the 

 most minute distinctions of form and colour become visible. 



In this case, the sensibility of the whole of the retina to 

 the three colours which form white light, is impaired by 

 the action of the strong light to which it was previously 

 exposed, and objects which reflect light of a much lower 

 intensity, are consequently invisible until the former im- 

 pression is removed, and the eye assumes a state suitable 

 to the new circumstances under which it has to perform 

 its office. 



If the light to which the eye was previously exposed had 

 been of one of the primitive colours only, the objects within 

 the room would have been immediately visible in the two 

 other colours, or if its sensibility had been impaired by the 



* The motion of the eye-lids enables us to see colours distinctly for a longer 

 time than ^hen this motion is suspended. 



