72 



Sir W. de W. Abney. 



retina. If the rods and cones fill the places in reference to the light sensa- 

 tions which have been allotted to them, as stated in my last communication 

 on the fourth sensation, then there is an absence of sensitive rods in the whole 

 retinae of the night-blind. The same remarks may apply to the observer B. 

 and his measures, though they differ slightly in shape for those of E., but 

 not more so than do the curves of various observers in the ' Philosophical 

 Transactions' paper just referred to. 



On May 20, 191.3, B. came for his examination at the colour laboratory. 

 He is a clergyman, and is, we believe, a cousin of E. His examination was con- 

 ducted on the same lines as that of E., the arc light alone being used. In 

 addition, he was made to match the colour of the D sodium line with mixed 

 colours of thallium and lithium blue, in what is called the anomaloscope, a 

 very iiseful instrument, though only partially indicative of any defect in 

 colour vision. 



[In Colonel Watson's and my paper, jointly published in the ' Philosophical 

 Transactions,' it was shown that there seemed to be two classes of retina : 

 one in which only the colours of the spectrum were recognised in the 

 spectrum and no colourless rays affected the fovea, and the other class in 

 which both coTbur and the feeble colourless rays were equally effective. As 

 we soon gathered the retinae of the two patients at any part apparently were 

 not stimulated by the colourless rays, we determined to compare them with 

 the No. I class of retina, for which purpose we had to reduce the arc results 

 into the N'ernst coloured spectrum. This was done, and the night-blinds' 

 extinction of colour coincided practically with the threshold of No. I retina 

 at the fovea.] 



It should be mentioned that Mr. Nettleship divides congenital night- 

 blindness into two classes, both showing the same night-blindness. The one 

 is myopic, but the other apparently normal, as far as refraction is concerned. 

 The two observers appear to be in the first division. 



The night-blind luminosity curves, taken with a luminous spectrum of the 

 arc, compared with that of the normal eye, show that the intensity of colour 

 is the same for both. Allowing for the difficulties of the shadow test, and 

 a certain error in observation which is found in the flicker test, the curves 

 of luminosity of both may be said to be the same. The luminosity curves 

 can be best compared by making the night-blind measures the numerator 

 and the normal measure the denominator of a fraction. The following are 

 the fractions for E. and B. : — 



