Thus, for instance, Dr. Whewell could not distinguish between red and green. But what colour did he really see? Did he see the leaves and cherries both red or both green, or was it some colour between the two that was impressed upon his retina? Again, great care must be exercised in placing implicit reliance on the statement of persons who are colour-blind, for we must recollect that their only means of conveying the results of what they experience is by the use of an organ that is confessedly defective, and which is quite likely to deceive them, and us too, without their being parties to the deception.
The cause of colour-blindness is completely unknown; philosophers and physiologists are still in the realms of hypothesis concerning this peculiar optical defect. As yet, the most careful observation has failed to detect any difference between the eyes of those who are colour-blind, and the eyes of ordinary persons, that could in any way account for this singular affection of the sense of sight.
