to particular Colours, 155 



philosopher who describes it, *' the eyes of an eminent optician, 

 whose eyes have this curious peculiarity, and have satisfied 

 ourselves, contrary to the received opinion,* that all the pris- 

 matic rays have the power of exciting and affecting him with 

 the sensation of hght, and producing distinct vision, so that 

 the defect arises from no insensibility of the retina to rays of 

 any particular refrangibility, nor to any colouring matter in the 

 humours of the eye, preventing certain rays from reaching the 

 retina (as has been ingeniously supposed,) but from a defect 

 in the sensorium, by which it is rendered incapable of appre- 

 ciating exactly those differences between rays in which their 

 colour depends. The following is the result of a series of trials 

 in which a succession of optical tints produced by polarized 

 light passing through an inclined plate of mica, was submitted 

 to his judgment. In each case, two uniformly coloured circu- 

 lar spaces, placed side by side, and having complementary tints,. 

 (that is, such that the sum of their hght shall be white,) were 

 presented, and the result of his judgment is here given in his 

 own words. 



Colours to an ordinary eye. Colours to Mr 's eye. 



Pale green, No colour. 



Dirty white, Darker, but no colour. 



Fine bright pink, Very pale tinge of blue. 



White, Yellow. .mv^v/Oii/A 



Rich grass-green. Yellow, but more coloured. 



Dull greenish-blue. Blue, 



Purple, rather pale. Blue* 



Fine pink. Yellow, with a good deal of blue^ 



Fine yellow. Good yellow. 



Yellowish green. Yellow with a good deal of blue. 



* We were not aware that any eyes had ever been regarded as absolutely 

 insensible to the luminous effect of any particular rays, but only to the co- 

 lorific effect of these raye, though we admit that the language used in trying to 

 explain the peculiarity may bear this construction. When we have spoken 

 of an eye insensible to red light j we meant only insensible to the redness of 

 light. 



It should be stated, however, that T. B. the subject of Mr Harvey's ob- 

 servations, {Edin. Trans^ vol. x. p. 253,) regarded indigo and Prussian 

 blue as black) and ako some greens as blacky that is, certain blue and 

 green rays made scarcely any impression on his retina. — Ed. 



