The portion of the electromagnetic continuum we normally call the Visible Spectrum begins with Red at 700 nanometers and ends at Violet with a wavelength of 400 nm.  In the Visual Spectrum, the 12th octave numbered 11, also begins at 700 nanometers, but continues on past Violet (400 nm) to end at the next Red, which is at 350 nanometers.

There is a gap between the Blue (Violet) at a wavelength of 400 nanometers, and the the Red at the end of the octave with a wavelength of 350 nanometers. The color that occupies this gap is invisible. What is this invisible color?

Remember when we mixed Blue (Violet) and Red to get Magenta? We found there was no place in the spectrum to put it. Now there is.  Magenta fits into this gap perfectly,- its wavelength being 373 nanometers, midway between Violet₁₁ and Red₁₂. Magenta's fundamental wavelength is then: 763.9 micrometers, 11 octaves below its discovered location.