The hypothesis of dark matter has an elaborate history. In the appendices of the book Baltimore lectures on molecular dynamics and the wave theory of light where the main text was based on a series of lectures given in 1884, Lord Kelvin discussed the potential number of stars around the Sun from the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20 to 100 million years old. He posed what would happen if there were a thousand million stars within 1 kiloparsec of the Sun (at which distance their parallax would be 1 milliarcsecond). Lord Kelvin concluded: