The Perception of Colour. 29 



more intense after a night's sleep than later in the day ; 

 and in this may be found an explanation of the great 

 sensitiveness to light of an eye which has been long in the 

 dark. The transparent retina has become more fully satu- 

 rated with the pigment, and more tumultuous chemical 

 changes go on, with correspondingly intense stimulation of 

 the optic nerve. This varying sensitiveness of the retina at 

 different times of the day has been made the subject of 

 exact experiment by M. Auguste Charpentier (Academy of 

 Sciences, 20th May, 1878, v. Gazette Medicare, 23, 1878). 

 He found that the difference of acuteness in the rested and 

 active eye holds good with all kinds of light. For instance, 

 the eye which has been kept dark for 15 to 20 minutes 

 experiences a luminous sensation, with a minimum of green 

 light equal to 16, while the eye which has been, active 

 requires a minimum of 121 ; the comparative amounts of red 

 light under the same conditions being 12 and 50, and of blue 

 16 and 400. As Charpentier argues, it is impossible to 

 conceive of this difference of sensibility being due to fatigue, 

 in any proper sense of the word, since the eye which had 

 been in exercise had merely been performing its normal 

 function. The explanation, as he says, is to be found in 

 the comparative amount of retina-purple under the different 

 conditions investigated by him, the sensitiveness to light 

 being in direct proportion to the chemical changes in the 

 pigment produced by that light. In a further note 

 to the Academy (27th May, 1878, Gazette Medicale, 24, 

 1878), M. Charpentier reported that according to his 

 direct observations it seems to result, that where there is 

 less of the red substance in the retina there is less 

 luminous sensibility, and that when the red is in excess that 

 sensibility is exaggerated. These facts taken together seem 

 to put beyond doubt that the retina-purple plays a very im- 

 portant, perhaps essential, part in the physiology of vision. 

 When we proceed to apply the knowledge recently gained 

 in a more special way, difficulties increase. I propose, how- 

 ever, to consider in how far the discoveries of Boll and 

 Kiihne throw light on the very difficult question of the 

 perception of colour, and before doing so it is necessary to 

 indicate shortly the generally accepted view on that subject. 

 Early in the present century Dr. Thomas Young proposed a 

 theory which has been, with slight modifications, adopted by 

 Helmholz, and accepted generally by physiologists. It is to 

 the effect that in every spot of the retina capable of receiving 



