Two Cases of Congenital Night-Blindness. 



71 



The clinical aspects of night-blindness, of course, my colleague and 

 myself are unable to discuss. It suffices to say, I think, that these two 

 cases fulfilled the object we had in view, viz., to examine two of the con- 

 genital cases which (the late) Mr. Nettleship included in one of his pedigrees. 



The first patient, Mr. E., a clergyman, came to the laboratory on 

 February 25, 1913, when he was examined in the writer's apparatus and 

 presence for his extinction of colour from the red to the blue of the arc spectrum. 



The luminosity of the D line coming through slits to the screen was the 

 standard of the intensity of the luminosity of the different rays of the 

 spectrum. This was gradually diminished by an annulus placed in the path 

 of the ray until he just saw no light in the illuminated small square in the 

 darkened camera attached to the spectroscopic apparatus. He said that 

 all light had vanished when the colour was extinguished. He repeated the 

 observations in a reverse manner, noting the advent from darkness of the- 

 spectrum colour. 



This series of observations was repeated in a second apparatus in which 

 the source of light was a ^Ternst lamp, the thread of which was rendered 

 incandescent by an ampere of current from a battery of 100 volts; the 

 current was kept constant. Similar observations were made by this apparatus 

 and recorded, and afterwards converted to the arc scale. Finally, the 

 luminosity curve of his spectrum was taken and compared at the time with 

 that of my colleague. 



In his observations he invariably said that, when the extinction was noted' 

 by the reduction of the said intensity, whenever the colour was gone all the 

 light had also gone. In other words, the same reduction in intensity of the 

 light was the threshold for both light and colour. In the paper, " The 

 Threshold of Vision for Different Coloured Lights,"* the same identity of 

 extinctions of light and colour at the fovea of Class I retina is to be noted, 

 as given by all observers, though the extinction of light outside the fovea is 

 nearly O^OOOl times less than it is for colour at the fovea. 



Comparing the loss of colour for my own eye (Class II) when the intensity 

 of the spectrum D is one candle-foot, the colour is extinguished at an intensity 

 of about 0'0016 candle-foot, and the extinction of light at about 0'000035 candle- 

 foot intensity. For " E." the extinction of both light and colour in both cases 

 takes place at about 0"0015-6. This indicates that the extinction of the 

 feeble white light (or, as I have called it, of the fourth colourless sensation) 

 is dependent on some other retinal perception that the normal eye possesses 

 beyond that possessed by the night-bHnd. The same is probably the cause 

 of the difference in the fovea of No. I retina compared with that of Xo. II 



* ' Phil. Trans.,' A, vol. 216. 



