512 



Capt. W. de W. Abney. On the Limit of [May 14 r 



and the other as just described, gave identical results within th 

 limits of the errors necessarily due to observation. 



The method adopted gave the extinction of light on the whole 

 retina, for not only was the central part used, but the extinction wi 

 carried so far that it was complete for every part of the eye. AM 

 there is a considerable absorption in the yellow spot this is necessary, 

 but the absorption exercised in this part of the eye, which occupies 

 from 4 to 6 angular aperture, can be fairly measured if only the 

 light on a small area be extinguished and this part of the retina 

 be alone used. A very simple way of seeing the absorption of the 

 yellow spot is to form a feeble spectrum some 3 inches long on 

 ground-glass screen. If the eye looks at the green, a dark band ex- 

 Table I. 



