434 ON THE THEORY OF COMPOUND COLOURS. 



and brighter than the mirror, shewing that the parts of my eye at a distance 

 from the axis are more sensitive to this blue-green light than the parts close 

 to the axis. 



It is to be noticed that this insensibility is not to all light of a green 

 or blue colour, but to light of a definite refrangibility. If I had a species of 

 colour-blindness rendering me totally or partially insensible to that element of 

 colour which most nearly corresponds with the light in question, then the light 

 from the mirror, as well as that from the prism, would appear to me deficient 

 in that colour, and I should still consider them chromatically identical ; or if 

 there were any difference, it would be the same for all colours nearly the same 

 in appearance, such as those just beyond the line F, which appear to me quite 

 bright. 



We must also observe that the peculiarity is confined to a certain portion 

 of the retina, which is known to be of a yellow colour, and which is the seat 

 of several ocular phenomena observed by Purkinje and Wheatstone, and of the 

 sheaf or brushes seen by Haidinger in polarized light ; and also that though, 

 of the two observers whose results are given here, one is much more affected 

 with this peculiarity than the other, both are less sensible to the light between 

 E and F than to that on either side ; and other observers, whose results are 

 not here given, confirm this. 



XV. Explanation of the Differences between the two Observer*. 



I think, therefore, that the yellow spot at the foramen centrale of Soemmering 

 will be found to be the cause of this phenomenon, and that it absorbs the rays 

 between E and F, and would, if placed in the path of the incident light, 

 produce a corresponding dark band in the spectrum formed by a prism. 



The reason why white light does not appear yellow in consequence, is that 

 this absorbing action is constant, and we reckon as white the mean of all the 

 colours we are accustomed to see. This may be proved by wearing spectacles 

 of any strong colour for some time, when we shall find that we judge white 

 objects to be white, in spite of the rays which enter the eye being coloured. 



Now ordinary white light is a mixture of all kinds of light, including that 

 between E and F, which is partially absorbed. If, therefore, we compound an 

 artificial white containing the absorbed ray as one of its three components, it 



