8 REPOXIT — 1855. 



illumination to their extieniities. The general proposition, that the colours of the 

 spectrum are changed by absorption, was denied, as already stated, by Mr. Airy, 

 and by Dr. Draper and M. Melloni, whereas both M. Helmholtz and M. Bernard 

 have admitted it as an indubitable truth. In direct contradiction of Mr. Airy's 

 statement, M. Helmholtz has candidly remarked, 'that the changes of colour which 

 Sir D. Brewster described, as produced by absorption, are for the most part suffi- 

 ciently striking to be observed without difficulty ; ' and he adds, 'that a careful 

 repetition of at least the most important of the experiments, carried out in exact 

 accordance with the method laid down, and with every precaution taken, has, 

 indeed, taught him that the facts are described with perfect accuracy.' In 

 these words, which are those of M. Helmholtz himself, the change of colour is 

 admitted as a physical fact ; but he ascribes it to two causes : — 1, to the possible 

 admixture of rays scattered from the prism, and the other transparent bodies used 

 in the experiment ; and 2, to the mixture of complementary colours, produced by 

 the action of the other colours of the spectrum on the retina." 



The author remarks, that the first of these causes, namely, the possible admixture 

 of scattered rays, is a very extraordinary one, and that it should not have been 

 assumed without some attempt to show its probability. He observes, " it is cer- 

 tainly possible that scattered rays may have influenced my retina ; but, even if 

 such rays did exist, it would be necessary to show that they were the precise rays 

 which were capable of producing the alleged change of colour. Now M. Helmholtz 

 has not even attempted to make it probable that such disturbing rays exist or could 

 have influenced any retina if they did exist ; nor has he attempted to show that 

 such possible rays are of colours which are complementary to those which I saw. 

 With regard to the second cause, namely, the admixture of complementary colours, 

 I unhesitatingly deny that it had any influence in the pheenomena which 1 

 observed ; and I earnestly request the attention of the Section to the following 

 observations : — If the subjective perception of colour, when we view the spectrum 

 or make experiments, in which more than one colour reaches the eye, is capable 

 of altering the colours under examination, then all that has been written on colours, 

 thus seen, must be erroneous, and all the gay tints of Art or of Nature, which 

 we admire and study, are but false hues under the metamorphosis of a subjective 

 perception. We must not now pronounce a rose to be red and its leaves green 

 till we have stared at them through a chink or torn them from their footstalk. 

 The changes of colour by absorption which I have described I have distinctly seen, 

 and seen as coirectly as Newton saw his seven colours in the spectrum, and Hooke 

 his composite tints in the soap-bubble ; and, now that my eyes have nearly finished 

 their work, I cannot mistrust, without reason, such good and faithful servants. 



"The observations of M. Bernard, who has repeated only a few of my experiments, 

 differ very little in their character from those of M. Helmholtz. He maintains that 

 the conversion of the blue space into violet, which I observed, arises from the 

 diminution of the light by absorption. Now, if the colours of the spectrum thus 

 change when they become fainter, we would desire to know at what degree of illu- 

 mination we are to see the prismatic spectrum in its true colours. If the blue space 

 is converted into violet by the diminution of its light, then colour does not depend 

 upon refrangibility alone, but also upon intensity of illumination ; a doctrine as sub- 

 versive as mine of the opinion of Newton, that to the same refrangibility always 

 belongs the same colour. If M. Bernard's experiments be correct, it is perfectly 

 compatible with my opinion, because it only proves that the blue rays, when enfeebled, 

 lose their power over the retina sooner than the red. Na^^, it is a sound argument 

 in favour of the doctrine which it is brought forward to disprove." 



In concluding his communication, the author mentions that none of the oppo- 

 nents of the triple spectrum have repeated his fundamental experiment made with 

 an apparatus which he believes no person but himself possesses. He examines a 

 pure spectrum divided into compartments by the action of thin plates of calcareous 

 spar passing across a prism of the same suhstance. Each of these luminous com- 

 partments shades off into the adjacent dark spaces, and is in a different condition 

 from the corresponding portion of the complete spectrum. When the proper 

 absorbing media are applied to certain portions of this divided spectrum, he insulates 

 a large portion of white light indecomposable by the prism, and it stands beside a 



