156 HEVIEWS — EESEA-ECHES ON COLOUR-BLINDNESS. 



ioishes in proportion to the general abundance of colouring matter tinting the 

 choioid; and it would be highly interesting to know whether the fair-eyed and 

 the dark-eyed — apart from colourblindness — attach a different chromatic value to 

 the same eolour. The proverbial difference betweeu the tints preferred for dress 

 by blondes and brunettes, and the great fouduess of the negro races for white and 

 the primary colours, are probably in part, at least, related to differences in the 

 colour of the choriod, to which that of the hair and of the iris is a clue. The 

 hair is probably the more important external index of the chromatic condition of 

 fche choroid, especially where the hair differs in shade from the iris; but this is 

 not certain ; ana even if it were, it will often be impossible, in the living human 

 subject, to look at both, so that in all eases each should be examined, and the re- 

 cult recorded." 



There remains no other hypothesis to fall back upon hut that adopt- 

 ed by HerscheL, "Wartmann, Kelland, and others; namely, that 

 colour-blindness consists in an inability of the sensorium to distin- 

 guish between the vibrations produced by certain rays having different 

 wave-lengths. This, translated out of the language of the Undula- 

 tory theory, merely asserts that the eye of the colour-blind is in- 

 capable of distinguishing between certain colours, and whatever may 

 be thought of its value as an explanation, it certainly possesses the 

 merit tlxat it cannot be objected to. 



In fact, the whole subject of colour is to this day the grand 

 stumbling-block in the Science of Optics. We have on the one hand, 

 the objective phenomena of natural coloration ; on the other, the sub- 

 jective requirements of the TJndulatory theory, meeting in the pris- 

 matic spectrum as common ground. The facts of the former are still 

 unmeasured and unclassified ; the analysis of the latter, even in the 

 hands of Fresnel and Cauchy, fails to give satisfactory account of 

 Dispersion. The Experimentalist has to tell us how to measure the 

 intensity and hue of a given colour, and in what way a compound of 

 colours may produce to the eye the same impression as a single ho- 

 mogeneous one ; the analyst has to tell us what is the law which con- 

 nects the wavelength of a ray with its velocity of transmission in 

 different media. Till both are told, Theory and Experience will be 

 like the Youth and the fair Lily in Goethe's Tale, vainly striving to 

 come together. 



Perhaps one of the most important steps taken in this direction 

 for a long period is the invention by Mr. Maxwell of the instrument 

 which he calls a eolour-top ; the account of this beautiful contrivance, 

 far superior to the methods of Newton and Young, to the diagram 

 of Professor Forbes, and the pretty Chromascope of M. Soleil, we 

 extract from this work, where it was published for the first time. 



" Mr. Maxwell employs a disc of pasteboard, or metal, provided with a spindle, 

 so as to admit of it being spun as a top or teetotum. The spindle is in two pieces, 



