[From the Report of the British Association, 1856.] 



XII. On the Unequal Sensibility of the Foramen Centrak to Light of 



different Colours. 



WHEN observing the spectrum formed by looking at a long vertical slit 

 through a simple prism, I noticed an elongated dark spot running up and down 

 in the blue, and following the motion of the eye as it moved up and doicn 

 the spectrum, but refusing to pass out of the blue into the other colours. It 

 was plain that the spot belonged both to the eye and to the blue part of the 

 spectrum. The result to which I have come is, that the appearance is due to 

 the yellow spot on the retina, commonly called the Foramen Centrale of Soem- 

 mering. The most convenient method of observing the spot is by presenting 

 to the eye in not too rapid succession, blue and yellow glasses, or, still better, 

 allowing blue and yellow papers to revolve slowly before the eye. In this way 

 the spot is seen in the blue. It fades rapidly, but is renewed every time the 

 yellow comes in to relieve the effect of the blue. By using a Nicol's prism 

 along with this apparatus, the brushes of Haidinger are well seen in connexion 

 with the spot, and the fact of the brushes being the spot analysed by polarized 

 light becomes evident. If we look steadily at an object behind a series of bright 

 bars which move in front of it, we shall see a curious bending of the bars as 

 they come up to the place of the yellow spot. The part which comes over the 

 spot seems to start in advance of the rest of the bar, and this would seem to 

 indicate a greater rapidity of sensation at the yellow spot than in the surround- 

 ing retina. But I find the experiment difficult, and I hope for better results 

 from more accurate observers. 



