Hypothesis of Colour Vision with Mechanical Illustrations. 347 



Herings Theory tested. — On Hering's Theory, in addition 

 to red, green, and violet, there are also yellow, black, and 

 white sensations. To test this, as regards the yellow, a 

 fourth responder was used of relative frequency to correspond 

 with yellow. Then with drivers representing blue and 

 yellow light, the effects shown in Photo 6 were obtained. 

 Here the amplitudes of responses are unequal but the yellow 

 predominates. It is difficult to believe that this would 

 correspond to the impression of white light or pink as was 

 the case in Photo 4, where only the red, green, and violet 

 responders were used. 



Fineness of Discrimination of Colour. — Dr. Edridge-Grreen 

 states that in the spectrum the eye can distinguish eighteen 

 to twenty-seven different regions each of which appears 

 monochromatic but different from its neighbours. It was 

 therefore necessary to test the present hypothesis as to the 

 adequacy of three responders to provide for a fineness of 

 discrimination of this order. This was done by starting 

 with the driving pendulum of period equal to that of the 

 middle responder representing the green. The effect of this 

 is shown in photo 7. The suspension of the driving pendulum 

 was then repeatedly lengthened by small steps till in 

 photo 18 it equalled that of the red responder. The inter- 

 mediate effects are shown in photos 8 to 17. The differences 

 of relative amplitudes of the responders are usually clearly 

 marked as we pass from plate to plate. Hence, there are 

 twelve states shown in about half the range of the spectrum, 

 which would give a possible twenty-four in the whole 

 spectrum, and this approaches the order of discrimination 

 stated by Dr. Edridge-Green as being actually accomplished 

 by the eye in the most favourable cases. A glance at some 

 of the photos (e. g. 7, 8, and 9) will show that still finer 

 discrimination than that actually exhibited would be possible. 

 So that, on the present hypothesis of vibrating responders of 

 only three frequencies, provision is made for a fineness of 

 discrimination quite equal to that known to exist. This is 

 on the supposition that the relative amplitudes of the 

 vibratory responses can be appreciated through the changes 

 occurring in the eye just as well as one can estimate 

 the relative dimensions exhibited on these photographic 

 prints. 



In the light of these results the present hypothesis of 

 colour vision may be compared with that put forward by 

 Dr. R. A. Houstoun ( ff A Theory of Colour Vision, '' Proe. 

 Roy. Soc. ser. A, vol. xcii. no. A 642, pp. 424-32). 



