PHYSIOLOQY OP VISION AND SENSATION OF COLOUR. 229 



and to settle what alterations are produced in the layer of 

 rods by the intensest possible operation of monochromatic 

 light (monochromatic dazzling). 



There are obviously still great difficulties in directly uti- 

 lising the results at present obtained for a theory of vision 

 and sensations of colour. One of the first questions which 

 arises is as to the significance of the green rods. Must we 

 really distinguish in the frog's retina two morphologically 

 and functionally different kinds of rods, the majority being 

 red and the minority green ? Or ought we not rather to 

 assume the fundamental identity of all the rods of the retina, 

 and consider the red and green rods only as appearances 

 produced in similar elements by changing physiological con- 

 ditions ? The circumstance that in the retina which has 

 been exposed to white sunlight no difference between the 

 rods is demonstrable, speaks in favour of the latter alterna- 

 tive, and seems to show that there exists only one kind of 

 these elements. The observations above communicated of the 

 increase in number of the green rods by green and blue light 

 might also be brought forward in support of this view. But 

 unfortunately it must be confessed that these latter observa- 

 tions must not yet be viewed as absolutely decided. For it 

 is highly probable, for many reasons, that the relation of 

 green to red rods in each single retina is not constant, but 

 differs in the various regions of the retina, in the centre and 

 the peripheral zones. But if this be the case, it becomes a 

 very doubtful problem to compare two retinae together as to 

 their relative richness in green rods, and therefore I dare 

 only speak with great reserve as to the actuality of the ob- 

 servations above mentioned on the increase of the green rods 

 in green and blue light. 



But so long as the significance of the green rods is not 

 cleared up, so long as we do not even know whether they 

 are found only in Amphibia, or occur also in the higher and 

 highest vertebrates, in mammals and especially in man, it 

 will be very difficult to utilise the above results for a theory 

 of sensations of colour. The first thing to be done is to 

 carry out a similar series of observations to those on frogs, 

 in an animal whose retina approaches man's as far as possible 

 in structure, i. e. a monkey. Perhaps we may then make 

 discoveries which stand in some simple relation to the facts 

 settled by subjective observation as to the sensations of colour 

 in the human retina ; and it might then be possible to deduce 

 a really true theory based upon this agreement. 



At present the following statements may be viewed as 

 confirmed. 



VOL. XVII. NEW SER. Q 



