542 Intelligence and Miscellaneous Articles. 



The observation was made so that a surface which contained 

 light of the wave-length 535 fi/u (thallium green) was always kept 

 unaltered, and then the quantity of light expressed in breadths of 

 the slit was determined which was necessary to attain the same 

 brightness with the various wave-lengths of the spectrum in 

 question. The reciprocal of this breadth of slit I call the value 

 of the brightness of the spectral light examined for the given 

 illumination. 



The lowest degree of illumination was very near the lower limit 

 of excitation, it was so feeble that the observer had to be at least a 

 quarter of an hour in absolute darkness to perceive it at all. If 

 we call the quantity of light 1, which was necessary to produce 

 this brightness in the standard green field, the other shades of 

 brightness used are given by the quantities of light 16, 256, 1024, 

 4096, 16384, 65336, and 262144. The last degree corresponds 

 about for my eye to the illumination under which a white paper, 

 lighted with 600 candles at a distance of a metre, appears when I 

 view it through a diaphragm of 19 inillim. aperture. 



The principal results obtained by the various observers may be 

 summed up in the following statements, in which all data refer to 

 the dispersion spectrum of gas-light : — 



1. For all observers (two trichromatic, a green-blind, and a red- 

 blind) the curve for the values of brightness had almost exactly 

 the same form for the darkest shade, and it was that which has 

 been observed by Donders, Hering, Dieterici, and myself for greater 

 shades of brightness in congenital monochromasy. Its maximum 

 was at about 535 fifx. 



2. As the brightness increases the maximum increases with 

 trichromatic persons, at first slowly, then more rapidly, and at 

 length very slowly again. In the highest shade used it is about 

 610 fifjL. 



3. So far as the observations extend, green-blind persons exhibit 

 the same phenomena as the trichromates. 



4. With red-blind persons the maximum moves at first towards 

 the long wave-length, but with mean illumination attains the 

 wave-length 570 /z/x, and remains here stationary for higher shades 

 also, so far as the observations extend. 



The statement in (1) was foreseen by Mr. E. Hering and the 

 adherents of his theory, and w T as also observed shortly before the 

 publication of our investigations. But that we are not justified in 

 seeing in it a proof of the correctness of this theory follows from 

 the observation, also made simultaneously by myself, that the 

 distribution of brightness in the spectrum in individual cases is 

 also unchanged, when by certain pathological processes the real 

 sensation of colour is completely lost and only the sensation black- 

 grey-white remains. — Wiedemann's Annalen, No. 3, 1892. 



