OPTICS. 



have also been called complementary 

 colours. Since a mixture of all the 

 colours of the spectrum forms white 

 light, it is obvious, that if one is left 

 out, the mixture of the remainder will 

 not be white but, some other tint. This 

 other tint is found to be nearly that 

 which corresponds to the centre of gra- 

 vity of all the other colours which are 

 left. So that if we arrange the colours 

 in a circle, as iny?^. 47, which is nothing 

 more than the prismatic spectrum bent 

 round till its two ends A, B meet, we 

 shall see that the centre of gravity of 

 the colours which remain after one co- 

 lour is omitted, must necessarily be 

 opposite to the omitted colour, that is, 

 the complementary colour is found in 

 the same way as the accidental colour. 

 Thus 



if the red is omitted, the centre of gra- 

 vity of the remaining arch will be in the 

 blue. We have inserted in the figure a 

 small strip of white, and another of black 

 opposite to it, in order to include all the 

 specimens of accidental colours ; but it 

 ought to be remarked, that the tints 

 thus formed from the common pris- 

 matic spectrum are not exactly those 

 which experiment gives. The colours 

 which ought to be used are those which 

 are found in a spectrum formed by a 

 prism, when the beam of light is larger 

 than the angular magnitude of the real 

 spectrum. In this case, the centre of the 

 spectrum will be white light, passing 

 on one side into pale yellow, yellow, 

 orange, and red, and on the other side 

 into greenish blue, indigo, and violet. 

 In such a spectrum there is no un- 

 mixed green, though we have inserted 

 it in the figure. 



From the preceding facts, the reader 

 will have no difficulty in understanding 

 the common theory of accidental co- 

 lours. When the eye is fixed for some 

 time on the red wafer, the part of the 

 retina on which, the red rays fall is 



strongly excited by their continued 

 action. Its sensibility to red light must 

 therefore be diminished, in the same 

 manner as the palate when long accus- 

 tomed to a particular taste ceases to 

 feel its impression. When the eye, 

 therefore, is turned from the red wafer 

 to the white paper, the excited portion 

 of the retina is insensible to the red 

 rays in the white paper, and, conse- 

 quently, sees that colour which arises 

 from the union of all the other rays but 

 the red, that is, a greenish blue. The 

 same explanation applies to all the 

 other colours. If the wafer is white 

 and placed upon a dark ground, the 

 accidental colour must be black ; be- 

 cause the enfeebled portion of the re- 

 tina is insensible, as it were, to all the 

 colours which compose white light. 

 When the wafer is black upon a white 

 ground, the portion of the eye upon 

 which the image falls, in place of being 

 enfeebled, is refreshed by the absence of 

 light, while the rest of the retina around 

 it is enfeebled by the white light of the 

 paper. Hence, when the eye is turned 

 upon a white ground, it will see a por- 

 tion whifer than the rest, so that the 

 accidental colour of black is white. 

 La Place's theory of accidental colours, 

 which we cannot pretend fully to un- 

 derstand, is given by Haiiy in the follow- 

 ing words : " He supposes that there 

 exists in the eye a certain disposition 

 in virtue of which the red rays com- 

 prised in the whiteness of the ground, 

 are, at the moment when they arrive at 

 that organ, in a manner attracted by 

 those which form the predominant red 

 colour of the circle, so that the two 

 impressions become blended into one, 

 and the green colour finds itself at 

 liberty to act as though it existed alone. 

 According to this method of conceiving 

 things, the sensation of the red decom- 

 poses that of the whiteness, and while 

 the homogeneous actions combine to- 

 gether, the action of the heterogeneous 

 rays which are disengaged from the 

 combination produces its effect sepa- 

 rately." * 



When a strong impression of white 

 light is made upon the eye, a succes- 

 sion of remarkable spectra is visible. 

 When the sun was near the horizon, M. 

 ^Epinus fixed his eye steadily upon it for 

 fifteen seconds. Upon shutting his eye 

 he saw an irregular, pale, greenish yel- 

 low image of the sun surrounded with 



* Dr. Gregory's translation of Hauy's Trade de 

 fhys. vol. ii. p. 424, 



