1835.] Composition of White Light. 181 



it, ascribes the cause of accidental colours to the diminution 

 in the sensibility of the retina, in consequence of its being 

 fatigued by a prolonged impression from a particular set of 

 rays, without the qualification that the second impression 

 should be more feeble than the first. 



3. Another theory proposed by Scherfi(er, considers the 

 accidental image as the consequence of the prolongation of 

 the feebler impression, produced by rays different from the 

 predominant colour of the object. 



4. De Godart advanced two theories in 1776, {Journal de 

 Physique, viii. i.) He endeavours to prove by experiment 

 that the scale of tones of vision is as follows, beginning with 

 the highest : Black, blue, green, red, yellow, white. He 

 supposes that, after looking at a coloured object, if w^e cast 

 the eye upon a white ground, the continued direct impres- 

 sion produced by the object, acts upon the sensation from 

 the white, so as to lower the tone, and, this diminution is in 

 proportion to the elevation of the original impression. For 

 example, if we look at a red object, as the red impression 

 which remains on the eye is raised, according to the scale, 

 three tones above the black, it will lower the white the 

 same number of tones, which will reduce it to green, the 

 accidental colour of red. 



5. The second theory of De Godart appears to be a modi- 

 fication of that of insensibility. It supposes that a fibre, 

 acted upon by one object, remains incapable of communi- 

 cating the sensation of another, as long as it preserves the 

 impression of the first ; and that the different colours being 

 expressed by portions of the same fibre, which are propor- 

 tionally short according to the vividness of the tone ; that 

 part which has not acted, being excited by the white, pro- 

 duces the accidental colour. That is, if we look at a red 

 object, and cast the eyes upon a white ground ; then, 

 according to De Godart, the only portion of the fibre which 

 has not been acted on, will be excited to vibrate by the 

 white, and will produce the sensation of green. If the red 

 impression affects the eye, red and green, that is, white 

 will be perceived ; or, in other words, no accidental colour 

 will be seen. 



6. Darwin adopted the principle that the retina becomes 

 insensible, in consequence of fatigue from the action of the 



