THE SENSATION OF SIGHT. 219 



the retina is most extensive on the inner or nasal side of the 

 field of vision ; and according to recent researches of Woinow, 

 there is at the furthest limit of the visible field a narrow zone 

 in which all distinction of colours ceases and there only remain 

 differences of brightness. In this outermost circle everything 

 appears white, grey, or black. Probably those nervous fibres 

 which convey impressions of green light are alone present in 

 this part of the retina. 



In the second place, as I have already mentioned, the middle 

 of the retina, just around the central pit, is coloured yellow. 

 This makes all blue light appear somewhat darker in the centre 

 of the field of sight. The effect is particularly striking with 

 mixtures of red and greenish-blue, which appear white when 

 looked at directly, but acquire a blue tint when viewed at a 

 slight distance from the middle of the field ; and, on the other 

 hand, when they appear white here, are red to direct vision. 

 These inequalities of the retina, like the others mentioned in 

 the former essay, are rectified by the constant movements of 

 the eye. We know from the pale and indistinct colours of the 

 external world as usually seen, what impressions of indirect 

 vision correspond to those of direct ; and we thus learn to judge 

 of the colours of objects according to the impression which they 

 would make on us if seen directly. The result is, that only 

 unusual combinations and unusual or .special direction of atten- 

 tion enable us to recognise the difference of which I have been 

 speaking. 



The theory of colours, with all these marvellous and com- 

 plicated relations, was a riddle which Goethe in vain attempted 

 to solve; nor were we physicists and physiologists more suc- 

 cessful. I include myself in (the number ; for I long toiled at 

 the task, without getting any nearer my object, until I at last 

 discovered that a wonderfully simple solution had been dis- 

 covered at the beginning of this century, and had been in print 

 ever since for anyone to read who chose. This solution was 

 found and published by the same Thomas Young 1 who first 

 showed the right method of arriving at the interpretation of 

 i Born at Milverton, in Somersetshire, 1773, died 18*2. 



