586 PHYSIOLOGY 



(2) Cmtr.ist, after images and adaptation are readily explained as follows: 

 While a stimulus falls on the retina, the three \isual sul stiu.ees which uere pr< \ 



in equilibrium with their breakdown products, are caused to take up a neu position 

 of equilibrium. On cessation of the stimulus however then- is a return to the old 

 p -sition, and therefore the impulses sent to the brain arc those which corn spor.d to a 

 sensation of an opposite character, thus causing a negative after image, ('ontrast is 

 explained by supposing that anabolism in one area is accompanied by a stimulus 

 to catabolism in the same area, but the effect is not sharply limited but tends to spread 

 for a short distance over surrounding areas, and thus causes a change in their equilibrium 

 point which is of an opposite nature to that of the stimulus which originates them. Thus 

 blue light falling on a part of the retina causes anabolism in that area, which is followed 

 by an increased tendency to catabolism. This process affects the surrounding ana. pro- 

 ducing the same change and therefore the same sensation as would a yellow in ;;gc. 

 Adaptation is explained as the taking up of a new equilibrium point, for one or all 

 of the three substances. 



(3) Colour blindness is explained as follows : Total colour blindness by the pi 



of the black -white substances only. Red -green blindness by the deficiency of this 

 substance, and yellow-blue blindness in a similar way. But we find by experiment that 

 there are two classes of red-green blindness, nairely those which are red deficient, 

 and those which are green. The Hering hypothesis is incapable -of explaining tleir 

 causation in its present form. 



EDRIDGE GREEN'S HYPOTHESIS states that the function of the rod* 

 secrete visual purple. This pigment under the action of light stimulates the ends of the 

 cones and causes them to send impulses to the brain which vary according to the 

 wavelength of the light and its intensity. The rods are on this view merely secretory 

 organs, and take no other part in vision. The impulses having reached the brain go 

 first to a light perceiving centre, and then to another especially developed for the 

 appreciation of colour. In this colour centre there are three separate mechanisms, A\ Inch 

 correspond roughly with the red, green and blue fundamental colours, but which may 

 respond to other frequencies than those to which they approximately correspond. 

 Suppose, for example, that a monochromatic yellow light is falling on the retina, then 

 it is absorbed by the visual purple, and thus stimulates the ends of the cones. These 

 then send up the optic nerve impulses which have a mean frequency correspoi id in^ 

 to yellow light, but at the same time contains impulses of other frequencies on either 

 side, to a degree which depends on their closeness with the mean. For example in 

 the above case, beside impulses of frequency of yellow light there are also some \\hich 

 correspond to green and red. These impulses having reached the colour centre .stimu- 

 late the red and green mechanisms respectively, while those corresponding to the yellow- 

 also stimulate these same mechanisms, but in proportion to the energy which each 

 receives. This view may now be examined in the light of experiment. 



(1) The results of colour mixture would appear to be explained by it with the exv. -p 



ion that the mechanisms in the colour cent re must have very definite mean frequencies, 

 lor otherwise mixed colours will not be able to match the whole of the spectral i 



(2) Simultaneous contrast and after images are explained by Edridge Green in a 

 way which I find difficult to understand ; it would therefore be best not to attempt to 

 discii 



(:{) Colour blindness was initially explained as being due to defective appreciation 

 m the colour perceiving centre. The shortening of the red end of the spectrum would 

 be due not to the ma bi lit y of t he retina to react to the stimulus, but to fault on the part 

 of the colour c.-nti. \\hen receiving the m-i \e impulses. A different explanation has 

 been offered by Houston, \\ho has recently elaborated the hypothesis. He states 

 that colour blindness is due to tl ,\- reaction mi the part of the retinal 



apparatus. \\ liieh causes the energy of the stimulus to be ^pread <\ei too \\ ide a range 

 of frequencies. If such \\erc the ea>e one would expect a low appfccia 1 ion of colour, 

 as is found in a number of examples of colour blindness, but there would be difficulty 



