158 Sir David Brewster on Colour Blindness or 



retina as one which might be admissible as a cause of colour 

 blindness ; but only on the supposition that the choroid coat 

 should prove to be the seat of vision. 



In treating of the various forms of colour blindness, Prof. 

 Wartmann considers the greatness of their number as not 

 giving support to the constitution of the solar spectrum oi equal 

 and coincident spectra of the three primitive colours. " C'est ce 

 nombre si grand des varietes du Daltonisme c\mme semble tifie 

 objection contre la preuve que Sir David Brewster a cru trou- 

 ver dans cette anomalie pour appuyer sa theorle de trois cou- 

 leurs elementaires (Edin. Journ. of Science*, N. S., torn. v. 

 p. 206; Bibl. Univ., tom. 1. p. 14<7). On ne peut pas dire que 

 *ie fait physiologique, et le principe d'optique sur lequel il fonde 

 son analyse du spectre sont parfaitemenl d'accord et se confir- 

 ment mutuellement.'" What Prof. Wartmann calls my theory 

 of three elementary colours is a fact as rigorously demonstrated 

 as any physical truth can be ; but if he does not admit it as 

 true over the whole length of the spectrum, he cannot avoid, 

 if he makes the experiments, admitting it as true over the 

 greater part of it ; and this is all that is necessary for my pre- 

 sent argument. 



In the year 1822, in a paper published in the Edinburgh 

 Transactions, vol. ix. p. 433, I first established the fact, that 

 in part of the spectrum different colours were superposed. In 

 1827, Sir John Herschel, whose experiments had previously 

 led him to an analogous result, remarks, in his Essay on Light, 

 "that this doctrine is not without its objections; one of the 

 most formidable of which may be drawn from the various af- 

 fections of vision," namely colour blindness, which he goes on 

 to describe. My principal memoir on the triple spectrum, 

 founded on an immense number of physical experiments, did 

 not appear till 1831 ; and though I should never have dreamt 

 of bringing to my aid, in such an inquiry, a physiological fact, 

 such as that of colour blindness, I found it necessary to re- 

 move, if possible, the objection drawn from it by so distin- 

 guished a philosopher as Sir John Herschel. My views needed 

 no other support than what they derived from direct experi- 

 ments with absorbing media applied to spectra, either entire 

 or cut up into bands by the interference of polarized and of 

 common light ; but when I believed that I had removed the 

 force of the objection, I was entitled to say that the views in 

 question derived that kind of support which must always be 

 gained by the removal of an objection. 



* The original memoir, of which an abridgement only appeared in the 

 works here cited, was published in the Edinburgh Transactions, vol. xii. 

 p. 135. 



