ON THE THEORY OF COMPOUND COLOURS. 435 



will be much more altered by the absorption than the ordinary light, which 

 contains many rays of nearly the same colour, which are not absorbed. On the 

 other hand, if the artificial light do not contain the absorbed ray, it will be 

 less altered than the ordinary light which contains it. Hence the greater the 

 absorption the less green will those colours appear which are near the absorbed 

 part, such as (48), (52), (56), and the more green will the colours appear which 

 are not near it, such as (32), (36), (40). And these are the chief differences 

 between fig. 4 and fig. 5. 



I first observed this peculiarity of my eyes when observing the spectrum 

 formed by a very long vertical slit. I saw an elongated dark spot running up 

 and down in the blue, as if confined in a groove, and following the motion 

 of the eye as it moved up or down the spectrum, but, refusing to pass out 

 of the blue into other colours. By increasing the breadth of the spectrum, the 

 dark portion was found to correspond to the foramen centrale, and to be visible 

 only when the eye is turned towards the blue-green between E and F. The 

 spot may be well seen by first looking at a yellow paper, and then at a blue 

 one, when the spot will be distinctly seen for a short time, but it soon dis- 

 appears when the eye gets accustomed to the blue*. 



I have been the more careful in stating this peculiarity of my eyes, as I 

 have reason to believe that it affects most persons, especially those who can see 

 Haidinger's brushes easily. Such persons, in comparing their vision with that 

 of others, may be led to think themselves affected with partial colour-blindness, 

 whereas their colour-vision may be of the ordinary kind, but the rays which 

 reach their sense of sight may be more Or less altered in their proportions by 

 passing through the media of the eye. The existence of real, though partial 

 colour-blindness will make itself apparent, in a series of observations, by the 

 discrepancy between the observed values and the means being greater in certain 

 colours than in others. 



XVI. General Conclusions. 



Neither of the observers whose results are given here shew any indications 

 of colour-blindness, and when the differences arising from the absorption of the 

 rays between E and F are put out of account, they agree in proving that there 

 are three colours in the spectrum, red, green, and blue, by the mixtures of 



* See the Report of tlie British Association for 1856, p. 12. 



552 



