The invention of the thermoscope has been attributed to Galileo, Santorio Santorio, Robert Fludd, and Cornelius Drebbel. The instrument displayed the variations in air density produced by the changes in temperature. With the increasing adoption of graduated scales, the thermoscope was transformed into the modern thermometer, capable of measuring the temperature accurately. In the mid-seventeenth century, the Accademia del Cimento developed the Florentine alcohol thermometer. Designed in different versions, the Accademia's instruments used a variety of graduated scales. The instruments were not, however, calibrated with uniform criteria. This made it impossible to compare measurements. In the eighteenth century, mercury was adopted as the thermometric substance of choice. Many thermometer scales were proposed. The most successful were Réaumur's eighty-degree scale, Celsius's centigrade scale, and Fahrenheit's scale. The temperatures of melting ice and boiling water were selected as the fixed points on the scales.