The Mystery of the Sensual Qualities

Democritus of Abdera wrote that:

1. All our knowledge about the world rests entirely on immediate sense perception.

This is easily granted by everybody.

2. This knowledge fails to reveal the relations of the sense perceptions to the outside world.

This is why our model of the outside world all lack sensual qualities.

This is not so frequently realized simply because the non-scientist has a great reverence for science and thinks that scientists can make out what other humans cannot.

A physicist will tell you that yellow light:

• is transversal electro-magnetic waves of 590 millimicrons wavelength.
• is a vibration that hits the retina giving the sensation of yellow.

We see light as an adaptation to the sun’s radiation.

We may further ask: Is radiation in the neighbourhood of wave-length 590 JlJl the only one to produce the sensation of yellow? The answer is: Not at all.

If waves of 760 JlJl, which by themselves produce the sensation of red, are mixed in a definite proportion with waves of 535 JlJl, which by themselves produce the sensation of green, this mixture produces a yellow that is indistinguishable from the one produced by 590 JlJl.

Two adjacent fields illuminated, one by the mixture, the other by the single spectral light, look exactly alike, you cannot tell which is which. Could this be foretold from the wave-lengths - is there a numerical connection with these physical, objective characteristics of the waves? No. Of course, the chart of all mixtures of this kind has been plotted empirically; it is called the colour triangle.

But it is not simply connected with the wavelengths.

There is no general rule that a mixture of two spectral lights matches one between them; for example a mixture of ‘red’ and ‘blue’ from the extremities of the spectrum gives ‘purple’, which is not produced by any single spectral light.

The said chart, the colour triangle·, varies slightly from one person to the other, and differs considerably for some persons, called anomalous trichromates (who are not colour-blind).

The sensation of colour cannot be accounted for by the physicist’s objective picture of light-waves.

Could the physiologist account for it, ifhe had fuller knowledge than he has of the processes in the retina and the nervous processes set up by them in the optical nerve bundles and in the brain?

I do not think so. We could at best attain to an objective knowledge of what nerve fibres are excited and in what proportion, perhaps even to know exactly the processes they produce in certain brain cells - whenever your mind registers the sensation of yellow in a particular direction or domain of our field of vision.

But even such intimate knowledge would not tell us anything about the sensation of colour, more particularly of yellow in this direction - the same physiological processes might conceivably result in a sensation of sweet taste, or anything else.

I mean to say simply this, that we may be sure there is no nervous process whose objective description includes the characteristic ‘yellow colour’ or ‘sweet taste’, just as little as the objective description of an electro-magnetic wave includes either of these characteristics.

The same holds for other sensations. It is quite interesting to compare the perception of colour, which we have just surveyed, with that of sound.

It is normally conveyed to us by elastic waves of compression and dilatation, propagated in the air.

Their frequency determines the pitch of the sound heard.

(N .B. The physiological relevance pertains to the frequency, not to the wave-length, also in the case of light, where, however,the two are virtually exact reciprocals of each other, since the velocities of propagation in empty space and in air do not differ perceptibly.)

The range of frequencies of ‘audible sound’ is very different from that of ‘visible light’.

It ranges from about 12 or 16 per second to 20,000 or 30,000 per second.

Those for light are of several hundred billions.

The relative range, however, is much wider for sound which has about 10 octaves.

Visible light has one octave.

Sound changes with the individual, especially with age.

The upper limit is reduced as age advances.

Sound is a mixture of several distinct frequencies that never combines to produce just one intermediate pitch such as could be produced by one intermediate frequency.

To a large extent the superposed pitches are perceived separately - though simultaneously - especially by highly musical persons.

The admixture of many higher notes (‘overtones’) of various qualities and intensities results in what is called the timbre, by which we learn to distinguish a violin, a bugle, a church bell, piano … even from a single note that is sounded.

But even noises have their timbre, from which we may infer what is going on;

Even my dog is familiar with the peculiar noise of the opening of a certain tin box, out of which he occasionally receives a biscuit. In all this the ratios of the co-operating frequencies are all-important.

If they are all changed in the same ratio, as on playing a gramophone record too slow or too fast, you still recognize what is going on. Yet some relevant distinctions depend on the absolute frequencies of certain components.

If a gramophone record containing a human voice is played too fast, the vowels change perceptibly, in particular the ‘a’ as in ‘car’ changes into that in ‘care’.

A continuous range of frequencies is always disagreeable, whether offered as a sequence, as by a siren or a howling cat, or simultaneously, which is difficult to implement, except perhaps by a host of sirens or a regiment of howling cats. This is again entirely different from the case of light perception.

All the colours which we normally perceive are produced by continuous mixtures; and a continuous gradation of tints, in a painting or in nature, is sometimes of great beauty.