Note 665 — Chapter IV
Reading edition
The normal spectrum is that depending exclusively upon wave-length—the fundamental constant given by nature as regards light. It is obtained by the interference of rays, in the manner first exemplified by Fraunhofer, and affords the only unvarying standard for measurement. In the refraction spectrum (upon which Kirchhoff's map was founded), the relative positions of the lines vary with the material of the prisms.