1892.] 071 the Sensitiveness of the Eye to Light and Colour. 611 



thrown above it. If the luminosity of this colourless spectrum be 

 measured we shall get the result stated. The curve obtained in this 

 way is in reality identical with the other curves. By these four 

 methods then we arrive at the conclusion that the last colour to 

 be extinguished is the sensation which when strong gives the sensa- 

 tion of violet, but which when feeble gives a blue-grey sensation. 



One final experiment I may show you. It has been remarked that 

 moonlight passing through painted glass windows is colourless on 

 the grey stone floor of a cathedral or church. 



We can imitate the painted glass and moonlight. Here is a 

 diaper pattern of different coloured glasses and by means of the 

 electric light lantern we throw its coloured pattern on the screen. 

 The strength of moonlight being known we can reduce the intensity 

 of the light of the lamp till it is of the same value. "When this is 

 done it will be seen that the pattern remains, but it is now colour-less, 

 showing that the recorded observations are correct, and I think you 

 are now in a position to account for the disappearance of the colour. 



I have now carried you through a series of experiments which 

 are difficult to carry out perfectly before an audience, but at any 

 rate I think you will have seen enough to show you that the first 

 sensation of light is what answers to the violet sensation when it is 

 strong enough to give the sensation of colour. The other sensations 

 seem to be engrafted on this one sensation, but in what manner it is 

 somewhat difficult to imagine. Whether the primitive sensation of 

 light was this and the others evolved, of course we cannot know. 

 It appears probable that even in insect life this violet sensation is 

 predominent, or at all events existent. Insects whose food is to be 

 found in flowers seek it in the gloaming when they are comparatively 

 safe from attack. Professor Huxley states that the greatest number 

 of wild flowers are certainly not red but more or less of a blue 

 colour. This means that the insect eye has to distinguish these 

 flowers at dusk from the surrounding leaves which are then of a 

 dismal grey ; a blue flower would be visible to us whilst a red flower 

 would be as black as night. That the insects single out these flowers 

 seems to show that they participate in the same order of visual sensa- 

 tions. I venture to think, without adopting it in its entirety, that 

 these results at all events give an additional probability as to the 

 general correctness of the Young-Helmholtz theory of colour vision. 

 Where the seat of colour sensation may be is not the point, it is only 

 the question as to what the colour sensations make us feel which the 

 physicist has to deal with. The simpler the theory, the more likely 

 is it to be the true one, and certainly the Young-Helmholtz theory 

 has the advantage over others of simplicity. 



[W. DE W. A,J 



