342 Dr. Gladstone on Iransmitted Liyht. [Feb. 6, 



jective origin. Difficulties of another character have also been 

 urged against Brewster's deduction, by Helmholtz and others, and 

 may be drawn from Maxwell's experiments. It is certain that 

 changes in the apparent colour of a particular ray may arise from 

 other causes than the absorption of one kind of light, while another 

 kind having the same angle of refraction is transmitted. Of these 

 may be enumerated ; — First, an actual change of refrangibility, as 

 in the cases of " fluorescence," so fully investigated by Professor 

 Stokes. Secondly, a difference in the impression on the sense, arising 

 from change of intensity. Thus blue, if very luminous, inclines to 

 white, if faintly luminous to violet ; and so the fore-mentioned note- 

 book designates the faint rays about F that were transmitted by the 

 red bell-glass, " lilac," and the speaker had observed the blue in the 

 prismatic spectra given by ammonio-sulphate of nickel, and by 

 tincture of lavender, gradually shading off into violet as the light 

 passed through deeper strata of liquid. The yellow of the solar 

 spectrum appears to occupy a considerable space, if the sun be 

 bright, but if diffuse daylight be examined, that space appears 

 orange and green, while the yellow is perhaps confined to a very 

 luminous line a little beyond D. It is not to be wondered at there- 

 fore that the green in the spectra of port wine and of citrate of 

 iron, appears to invade the space usually occupied by the yellow, 

 and the orange yellow. Yet in such cases the impression on differ- 

 ent eyes may be very different ; thus, in rehearsing the experiments 

 with the electric light at the Royal Institution, Dr. Gladstone had 

 seen the bright space beyond D transmitted by blue glass of a 

 decidedly green tint ; but Mr. Anderson had unhesitatingly called 

 it yellow, its proper colour. This difference of sensation, arising 

 from difference of intensity, was illustrated by the " Cercles chro- 

 matiques " of M. Chevreul, the first of which represents the bright 

 colours of the spectrum, in which that called " Jaune " is certainly 

 a beautiful yellow ; but the succeeding circles represent the same, 

 reduced by the admixture of various percentages of black, and 

 in them the '''Jaune" becomes green^ and so likewise does the 

 " Orange" where a very large proportion of black has been added. 

 A revolving disk, coloured black, on which had been fastened a 

 segment of bright yellow paper, appeared uniformly green when 

 set in rapid motion. Again, on one of Maxwell's colour tops was 

 fixed an outer circle of red, and an inner one, partly black and 

 partly orange ; when the top was spun the inner circle appeared 

 green. Thirdly, contrast will frequently change the apparent 

 colour of a particular ray. The result in the last experiment 

 was partially due to this cause, the outer circle of bright red 

 facilitating the sensation of its complementary colour green. Thus 

 the dim light between D and E in the spectrum of ammonio- 

 sulphate of nickel, with bright orange on one side and green on 

 the other, assumes a very indefinite tint. The very remarkable 

 prismatic image given by a solution of permanganate of potash in 



