COLOUB VISION. 275 



about a fifth of the distance from 6 towards F. This green has a wave length 

 of about 510 millionths of a millimetre by Ditscheiner's measurement. This green 

 is either the true primary green, or at least it is the nearest approach to it 

 which we can ever see. Proceeding from this green towards the red end of 

 the spectrum, we find the different colours lying almost exactly in a straight 

 line. This indicates that any colour is chromatically equivalent to a mixture of 

 any two colours on opposite sides of it, and in the same straight line. The 

 extreme red is considerably beyond the standard red, but it is in the same 

 straight line, and therefore we might, if we had no other evidence, assume the 

 extreme red as the true primary red. We shall see, however, that the true 

 primary red is not exactly represented in colour by any part of the spectrum. 

 It lies somewhat beyond the extreme red, but in the same straight line. 



On the blue side of primary green the colour equations are seldom so 

 accurate. The colours, however, lie in a line which is nearly straight. I have 

 not been able to detect any measurable chromatic difference between the extreme 

 indigo and the violet. The colours of this end of the spectrum are represented 

 by a number of points very close to each other. We may suppose that the 

 primary blue is a sensation differing little from that excited by the parts of the 

 spectrum near G. 



Now, the first thing which occurs to most people about this result is that 

 the division of the spectrum is by no means a fair one. Between the red and 

 the green we have a series of colours apparently very different from either, and 

 having such marked characteristics that two of them, orange and yellow, have 

 received separate names. The colours between the green and the blue, on the 

 other hand, have an obvious resemblance to one or both of the extreme colours, 

 and no distinct names for these colours have ever become popularly recognised. 



I do not profess to reconcile this discrepancy between ordinary and scientific 

 experience. It only shews that it is impossible by a mere act of introspection 

 to make a true analysis of our sensations. Consciousness is our only authority ; 

 but consciousness must be methodically examined in order to obtain any trust- 

 worthy results. 



I have here, through the kindness of Professor Huxley, a picture of the 

 structure upon which the light falls at the back of the eye. There is a minute 

 structure of bodies like rods and cones or pegs, and it is conceivable that the 

 mode in which we become aware of the shapes of things is by a consciousness 

 which differs according to the particular rods on the ends of which the light 



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