1 58 Two cases of Insensibility of the Eye to Colours. 



of all the rays in the spectrum, to a person whose sensorium is 

 incapable of appreciating those differences between some of the 

 rays on which their colour depends ? 



2. If the colour of all the rays thus united is white, that is, 

 if it makes the same impression on the defective sensorium as 

 a perfectly white body, how does it happen that white was seen 



by Mr at one time as yellow, at another time as bhie, 



and at a third time as white f 



3. If the union of all the colours is not white, but is a mix- 

 ture of blue and yellow, the only colours which the eye of Mr 



perceives, why is white seen different from a mixture 



of blue and yellow ? 



4. The sensorium of Mr is not only defective in the 



power of discriminating colours, but it wants the power of ap- 

 preciating the joint influence of the colours which it does dis- 

 criminate, or of discovering in combination a colour which it 

 discriminates when seen separately. Fine crimson, for exam- 

 ple, is described by Mr as blue^ z&ith a good deal of 



T^ellmv, which would be described by a common eye as green- 

 ish ; and in a rich grass green, no blue is recognized, but it 

 appears only yellow. 



An answer may be given to some of these questions by sim- 

 plifying the case. If the eye was devoid altogether of the 

 sense of colour, the spectrum would appear light at the point 

 of maximum yellow, shading gradually off to both extremities, 

 and exactly as it would do to a sound eye if shaded off with In- 

 dian ink. In this case it cannot be doubted that such a spec- 

 trum would appear white if thrown into a circle and whirled 

 rapidly round. 



If the eye recognized only one colour, such as yellow, the 

 spectrum would appear yellow in the middle, and shading off 

 as in the first case ; and if it were thrown into a circle and 

 whirled rapidly round the whole would be yellow. 



If the spectrum now consists of two colours which are 

 alone recognized, viz. yellow and blue, we know that thieir 

 unionwill not he green, for the eye is insensible to this tint ; we 

 cannot understand how it can be white-., and therefore we con- 

 ceive that the retina may be affected in some points with blue 

 and in others with yellow, an effect which may be produced in 



