Slide showing chart of 'rhythm of sex desire and frequency of conception from timed single coitus'. Slide shows chart of 'rhythm of sex desire and frequency of conception from timed single coitus'. 'X' axis shows days of menstrual cycle from day 23 through to day 3 of next cycle. There are two lines on the chart which are explained in the following key: 'Fig.4. Rhythm of sex desire (heavy line) and frequency of conception from timed single coitus (light line). [From Tinklepaugh, based on Dickinson's data.]' Late 19th to early 20th century