COLOUR VISION. 177 



light-waves and is roughly speaking grey, certainly not pure black, as it ought to 

 be as far as the Young theory goes. According to the Hering theory the equili- 

 brium of the white-black substance is the resting-state, associated with grey sen- 

 sation, and the black sensation belongs to active katabolism. 



When it comes to an explanation of partial colour-blindness this theory breaks 

 down. Absence of the red-green component would be the most obvious explana- 

 tion to advance but this does not hold in view of the two different types of the 

 defect. There are also difficulties arising from more detailed experiments on 

 after-images which stand in the way of accepting this theory as a complete 

 expression of the facts of colour vision. Indeed no theory has as yet provided 

 that, a fact not to be wondered at in view of the extreme complexity of the reac- 

 tions involved 



Experiment 71. Map out the extent of the colour fields on the 

 retina by using the perimeter. This consists of a strip of metal 

 bent to form an arc which can be rotated in various meridians. 

 The arc is graduated from the axis and it carries a small rider 

 to which a piece of white or tinted paper is attached. The 

 observed person rests his chin on the wooden pillar placed in 

 front of the arc, and with one eye closed he looks steadily at 

 a small white spot marked on the axis of the instrument. The 

 observer, moves the carrier from the free end to the axis 

 of the arc slowly, and notes the exact graduation at which the 

 subject first perceives it. This distance he then marks on a 

 chart which consists of a circle with various diameters repre- 

 senting the meridians each graduated to correspond to the arc, 

 from the centre outwards. 



In transferring the readings to the chart, allowance must be 

 made for the crossing at the nodal point of the eye. The obser- 

 vation is repeated for at least twelve different meridians. Having 

 mapped out the visual field for white (by joining the marks on 

 the chart) the observation is repeated using red, yellow and 

 blue papers on the carrier. 



