Prior to 1950, observations were conducted mainly using antennas that recorded the intensity of the whole Sun at a single radio frequency. Observers such as Ruby Payne-Scott and Paul Wild used simultaneous observations at numerous frequencies to find that the onset times of radio bursts varied depending on frequency, suggesting that radio bursts were related to disturbances that propagate outward, away from the Sun, through different layers of plasma with different densities. These findings motivated the development of radiospectrographs that were capable of continuously observing the Sun over a range of frequencies. This type of observation is called a dynamic spectrum, and much of the terminology used to describe solar radio emission relates to features observed in dynamic spectra, such as the classification of solar radio bursts. Examples of dynamic spectra are shown below in the radio burst section. Notable contemporary solar radiospectrographs include the Radio Solar Telescope Network, the e-CALLISTO network, and the WAVES instrument on-board the Wind spacecraft.