1857.] exhibited by Transmitted Light. 341 



green pass through a thin stratum of the mixed salts, while red alone 

 is transmitted by a thick stratum. 



Sir David Brewster observed that some coloured media caused 

 a ray of a certain angle of refraction to appear of a different colour 

 to that which it exhibited in the normal spectrum ; and, mainly on 

 these observations, he founded his remarkable theory that the pris- 

 matic spectrum consists of three superimposed spectra of the same 

 length, one red, another yellow, and the third blue, which are 

 coincident in position, but have their maxima of luminosity at 

 different places. Some eminent philosophers of our own and other 

 lands have denied not merely the conclusions, but even the obser- 

 vations of Brewster. Dr. Gladstone, however, could add his fullest 

 testimony to the truth of the statement, that absorbent media fre- 

 quently produce an apparent change of colour in a transmitted ray ; 

 and that not merely when a slit in the window- shutter is viewed by 

 a prism through the interposed medium, but also when the altered 

 prismatic spectrum is thrown upon a white screen. He had tried 

 the latter experiment by means of light derived from the sun, from 

 the electric lamp of the Royal Institution, and from the oxy-gas 

 lime lamp of Mr. Highley, with the kind assistance of that gentle- 

 man, and always with a similar result. The large bell-glasses used 

 during this discourse had been originally employed by Dr. Gladstone 

 for experiments on the growth of plants, and he had then carefully 

 examined the light transmitted through them. Through the blue 

 glass he saw first a band of pure red light, then a dark space, then 

 another luminous band which appeared to him like no colour of the 

 spectrum, rather russet perhaps ; his assistant called it " dirty 

 chocolate ; " a lady, who happened to come into the laboratory, 

 unhesitatingly pronounced it " orange ; " he was struck with this, 

 as it certainly corresponded in position with the orange ray, 

 though he did not know at that time, what has been frequently 

 observed, that women are generally far more accurate in their 

 appreciation of colours than men are. Accordingly he described 

 the luminous band in his note-book as " orange, very bright, but 

 unlike normal colour." Subsequently, he had found that Brewster 

 called this second red ray in smalt blue glass " orange red ; " but 

 Herschel pronounced it " pure red ; " while Helmholtz, isolating 

 it from surrounding light, resolved it into its proper orange. 

 Quite recently, on examining the prismatic spectrum thrown on a 

 screen after traversing the same kind of glass, one scientific friend 

 had called the second luminous ray "green," and another had 

 designated it " brown," though on reconsideration each indepen- 

 dently thought it had rather a reddish tint. 



Thus Brewster's observation that a ray after transmission through 

 certain absorbent media appears of a different colour to what it did 

 before, is a truth. Yet the very fact that this colour seems so dif- 

 ferent to different eyes, and indeed to the same eye at different times, 

 indicates that the phenomenon has a subjective rather than an oh- 



