86 On the Invisibility of certain Colours to certain Eyes. 



Mr Harvey has described, in a paper read before the Roy- 

 al Society of Edinburgh, and which will soon be published, 

 the case of a person now alive, and aged 60, who could dis- 

 tinguish with certainty only white, yellow, and grey. He 

 could, however, distinguish blues when they were light. Dr 

 Nichols has recorded a case where a person who was in the 

 navy purchased a blue uniform coat and waistcoat, with red 

 breeches to match the blue, and he has mentioned one case in 

 which the imperfection is derived through the father, and ano- 

 ther in which it descended from the mother. 



In the case of a young man in the prime of life, with whom 

 the writer of this article is acquainted, only two colours were 

 perceived in Dr Wo]laston"s spectrum of five colours, viz. red, 

 o-reen, blue, and violet. The colours which he saw were blue 

 and orange or yellow, as he did not distinguish these two 

 from one another. When all the colours of the spectrum 

 were absorbed by a reddish glass, excepting red and dark 

 green, he saw only one colour, viz. yellow or orange. When 

 the middle of the red space was absorbed by a blue glass, he 

 saw the black line with what he called the yellow on each side 

 of it. We are acquainted with another gentleman who has'a 

 similar imperfection. 



In all the preceding cases there is one general fact, that 

 red light, and colours in which it forms an ingredient, are not 

 distinguishable by those who possess the peculiarity in ques- 

 tion. Mr Dalton thinks it probable that the red light is, in 

 these cases, absorbed by the vitreous humour, which he sup- 

 poses may have a blue colour; but as this is a mere conjec- 

 ture, which is not confirmed by the most minute examination 

 of the eye, we cannot hold it as an explanation of the pheno- 

 mena. Dr Young thinks it much more simple to suppose the 

 absence or paralysis of those fibres of the retina which are 

 calculated to perceive red ; while Dr Brewster conceives that 

 the eye is, in these cases, insensible to the colours at the one 

 end of the spectrum, just as the ear of certain persons has 

 been proved, by Dr Wollaston, to be insensible to sounds at 

 one extremity of the scale of musical notes, while it is perfect- 

 ly sensible to all other sounds. 



If we suppose, what we think will ultimately be demon- 



