Intelligence and Miscellaneous Articles. 399 



We employed a method the principle of which had been indicated 

 by the latter, and which is that used in experimental physiology to 

 test the excitability of nerves. This method consists in determin- 

 ing the minimum of excitation necessary to be applied to the nerves 

 in order that they may react. Now light is the normal excitant of 

 the optic nerve, and the luminous sensation its special mode of re- 

 action. The question therefore was, to determine, for the centre 

 of the eye and for points more and more excentric, what is the 

 minimum of light that must be presented to the eye to obtain a 

 luminous sensation. But the eye perceives not only light, but also 

 colours ; so it was necessary to repeat for the monochromatic rays 

 the same experiments as for white light. 



To accomplish these two different ends we made use of a very 

 simple instrument, contrived by one of us *, consisting essentially 

 of a convex lens producing on a roughed plate of glass the image 

 of a luminous object. By utilizing, with the aid of a special dia- 

 phragm, various extents of the lens, images are obtained, always 

 distinct, but differently illuminated ; and their relative illumination 

 can be estimated ivith quite sufficient exactness from the extent of 

 lens employed to produce them. The luminous object consists of 

 another roughed glass plate of determined superficies, uniformly 

 illuminated with the kind of light, white or coloured, the effect of 

 which we wish to try. 



The experiment consists in viewing in the dark the image formed 

 by the object upon our first screen, while the eye fixes successively 

 the image itself and points more and more distant from it. For 

 each position of viewing, the observer determines what is the 

 minimum of luminous intensity necessary for our image to produce 

 a sensation of light or colour. 



Such is essentially the method we have employed for determining 

 the excitability of the different parts of the retina, aud which has 

 given the following results : — ■ 



Touching the purely luminous sensibility of the different points 

 of the retina, we have constantly found necessary, for the centre 

 and for each of those points, the same minimum of white light in 

 order to produce a luminous sensation. The luminous sensibility 

 is therefore the same for every part of the retina. 



It is otherwise if, instead of exciting the retina with white light, 

 we excite it with monochromatic light. Then it is seen that, in 

 order to distinguish the colour presented to the eye, we do not re- 

 quire that colour to possess so much intensity for the centre as for 

 the rest of the retina, and that, the further we remove from the 

 point of fixation (that is, from the centre), the more intense must 

 the colour be, in order to be recognized. 



But, a remarkable thing, before any colour is recognized with 

 its true tone it appears always to pass through a series of phases, 

 the first of which is interpreted by a purely luminous sensation ; 

 then one hesitates about the quality of the colour presented, until 

 the excitation has attained a certain intensity, with which the colour 

 * See Oharpentier, Societe de Biologie, Feb. 17, 1877. 



