1892.] on the Sensitiveness of the Eye to Light and Colour. 609 



eye. I would ask you to note that at each end of the spectrum only 

 one sensation is present, viz. at the red end of the spectrum, the 

 red sensation, and at the violet end the violet. 



This is a matter of some importance, as we shall now see. 



It will be recollected that in making the extinctions, the D light 

 of the spectrum was made equal to one amyl-acetate lamp, and the 

 other rays had the relative luminosity to it, which they had in the 

 spectrum before they were extinguished. The luminosity curve of 

 the spectrum is shown in Fig. 5. 



Suppose we make all the luminosities of the different rays equal 

 to one A L, we should not get the same extinction value, as shown 

 in the continuous lines in Fig. 3. The violet would have to bo 

 much more reduced, but by multiplying the extinction by the lumi- 

 nosity we should get the curve of reduction for equal luminosities, 

 and we get the dotted curves in Fig. 3. 



It will be seen that it is the violet under such circumstances that 

 would be the last to be extinguished, and that all the rays at the 

 violet end of the spectrum would be extinguished simultaneously, as 

 would also those at the extreme red. This looks like a confirmation 

 of the Young-Helmholtz theory which I have briefly explained, for 

 we cannot imagine that it can be anything but a single sensation 

 which fails to be excited. 



15 



The violet is extinguished when it is ^ — r A L, that is, a 



screen placed 817 feet away and illuminated by an A L violet lamp 



... 17 



would be invisible. The blue-green (F) light when it is ,-- — r- 



10 millionths 



35 



or 770 feet away. The green (E) light ^ q millionths ^^ ^^^ ^®^* 



350 

 away. The orange (D) light is extinguished as before at ^ — ^. — -, - 



or 180 feet away, whilst the red (C) light has only to be reduced to 



2200 

 jt: — ryT-. — TT- or an A L lamp radiating C light would have to be 



placed only 67 feet away, whilst tlie radiation for an A L of the 

 colour of the B light of the spectrum would have to be diminished to 



^^*To — TT — fh ^^ *'^® screen would have to be placed 60 feet 

 away. 



It is therefore apparent that with equal luminosities the violet 

 requires about 175 times more reduction to extinguish it than does 

 the red, and probably about 25 times more than the green. 



This being so, I think it will be pretty apparent that, at all events 

 from the extreme violet to the Fraunhofer line D of the spectrum, the 

 extinction is really the extinction of the violet sensation, a varying 

 amount of which is excited by the different colours. If then we take 



