74 The Astronomer Royal on Sir David Brewster's 



First, then, I remark that, supposing Sir David BreYi^ster's 

 alleged experimental facts to be placed beyond doubt, the infer- 

 ence as to the three colours is exceedinglydoubtful. It appears 

 certain that combinations of other colours might be made to 

 produce the apparent colours of the spectrum ; and there 

 seems therefore to be no sufficient reason for selecting those 

 adopted by Sir David Brewster. It is quite pi'oper that the 

 reader's attention should be called to this; for in consequence 

 of Sir David Brewster having given the details of his theory 

 in considerable extent before he gives the experiments, the 

 reader may be led to suppose that the experiments do actually 

 exhibit three such spectra as are implied in the title of the 

 paper. Those, however, who will enter into the minutiae of 

 the paper, will see that the experiments exhibit no such thing. 

 The simple facts which the experiments are supposed to ex- 

 hibit are, that by the use of different absorbing media, certain 

 portions of the spectra undergo certain changes of colour. 

 And the only question which I am disposed to entertain is 

 this: Is it established that the colour of any portion of the 

 spectrum is changed by the use of any absorbing medium? 



My second remark applies to what 1 conceive to be one 

 possible cause of error in Sir David Brewster's experiment. 

 It does not appear from his paper that the spectrum in its 

 modified state was compared immediately with an unmodified 

 spectrum. Now I state unhesitatingly as my own belief, that 

 the eye has no memory for colours ; and I have no confidence 

 in any evidence upon the change of colour, unless the colour 

 which is not altered and the colour in which change is sus- 

 pected are placed side by side. 



Thirdly, I remark that it does not appear that care was 

 taken to exclude light coming to the eye from other sources. 

 Indeed, in an experiment of Sir William Herschel's, to which 

 Sir David Brewster refers, it is certain that there was a mix- 

 ture of other light. When the spectrum was thrown upon clean 

 brass, "the colour of the brass made the red rays appear like 

 orange, and the orange colour was likewise different from what 

 it ought to be." But on referring to Sir William Herschel's 

 paper, it will be seen that the spectrum was formed by a prism 

 placed in the upper part of an open window; and it is'not 

 difficult to predict what must be the effect of the light coming 

 from all other parts of the window. 



I now proceed to describe the form in which my own ex- 

 periments were made. 



The windows of my lecture-room were darkened by closing 

 all the shutters, the joints of which were almost absolutely 

 impervious to light. Tlirough a hole in one shutter, a beam 



