Philip Newton scripsit:

> But there is no wavelength of light that is perceived as "yellow" by
> normally-sighted people: the point is that yellow light is made up of
> several wavelengths (prototypically by at least one wavelength that's
> red and one that's green).

Such light will indeed be seen as yellow, but there are also true
spectral yellows: for example, the yellow of a sodium-vapor lamp is
essentially monochromatic at 589 nanometers.  The range of single
frequencies perceived as yellow ranges from about 560 to 590 nm.

The fact that both spectral yellow and mixtures of spectral red and
spectral green are seen as yellow is a consequence of the response
curves of the red and green (really yellow-green) pigments in the eye.

Verbogeny is one of the pleasurettes    John Cowan <[log in to unmask]>
of a creatific thinkerizer.   
   -- Peter da Silva