136 Sir David Brewster 07i Colour Blindness or 



most 'perfect manner are insensible to certain impressions of 

 highly attenuated light which are quite j^^i^ceptible to other 

 eyes." Now it was a very natural conclusion that this insen- 

 sibility might be greater for some colours than for others, and 

 as Dr. Wollaston had just published his discovery of a similar 

 phaenomenon in reference to the ear*, and had ascribed it to a 

 difference in the state of the tympanum, I conceived that an 

 analogous view might be taken of colour blindness. Perhaps 

 Dr. Wollaston was too bold in ascribing cricket deafness^ as 

 it may be called, to the state of the tympanum, and not to a 

 defect in the se?isorium, and I also too bold in pursuing the 

 analogy from the tympajium to the retina. Dr. Wollaston has 

 not mentioned any case, though I have no doubt there are 

 many, in which cricket deafness is confined to one ear. Al- 

 though my own hearing is perfect, and each ear equally acute 

 for all ordinary sounds, yet one oi them is absolutely deaf to 

 the chirp of the cricket, while the other hears it distinctly. 

 Now it will not, I presume, be maintained that there is a sen- 

 sorium for each ear. In like manner, and I have no hesita- 

 tion in predicting it, there may be found persons whose colour 

 blindness is confined to one eye, or at least is greater in the 

 one eye than in the other. Nor is this wholly a conjecture from 

 analogy, for my own right eye, though not a better one than 

 the left, which has no defect whatever, is more sensible to red 

 light than the left eye. 



But there are still stronger points to be adduced in favour of 

 these views. I have proved from numerous experiments, that 

 when the retina is rendered partially insensible by the action 

 of light upon any one part of it, it first becomes insensible to 

 red light ; and hence we have a distinct reason why red-colour 

 blindness is the general character of the defect under consi- 

 deration ; and I am persuaded that any defect of sensibility 

 produced by the action of light or by any other cause, will, if 

 carefully examined, be found to be a maximum with red light. 



In experiments of this kind, in which what we may call «r- 

 tificial colour blindness is produced, the intensity of the light 

 is always diminished ; but it remains to be determined by ac- 

 curate observation whether the red end of the spectrum (for 

 example) when seen yelloiso by an eye defective in its judg- 

 ment of colours, is brighter or more obscure than it would have 

 been had no such defect existed. 1 am persuaded, from many 

 observations I have made, though I do not consider them as 

 decisive of the question, that the object is seen more obscure, 

 and that certain of the rays emanating from it are not appre- 

 ciated by the nervous membrane. If, on the other hand, every 



[* Dr. Wollaston's paper on this subject was reprinted in Phil. Mag. 

 S. 1. vol. Ivii. p. 187.] 



/ 



