- Visual observation is limited to the range of frequencies to which our eyes are sensitive.
The careful measurements of Kayser and Runge of the carbon bands show that the successive differences in the frequencies do (1900), I, p. 399.
Wolfer's frequencies with data obtained by other observers for areas of sun-spots, and his figures show unquestionably that the unit in one or other set of data must have varied appreciably from time to time.
The frequencies are nearly in the ratios 1: 6.25:17.5.
In another party line system a harmonic principle is employed: the ringing machines deliver alternating currents of four frequencies, while each bell is constructed to operate at a particular frequency only.