Image: Apparatus for Personal Equation

Description: From Pearson (1902), page 245: "Mr. (Horace) Darwin constructed for us a pendulum, consisting of a bar swinging on knife edges from an axis through its middle point. At either end of the bar were weights, so that by their adjustment very slow or very quick swings could be obtained. The pendulum could be released from rest at any angle from the vertical. Attached to the bottom of the pendulum was a small bell, which struck a very light hammer as it passed through the lowest point of the swing. This hammer was easily adjustable and was pulled upright by a string between each experiment, being knocked over by the transit of the pendulum. A mirror swinging about a horizontal axis had a strut attached to this axis and perpendicular to the plane of the mirror. This strut rested on a saddle (a) attached to a similar strut perpendicular to the pendulum bar at its axis. By shifting the saddle on the strut the mirror could be made to swing through a very small or a fairly large angle, whatever might be the amplitude of the pendulum. The whole object of this arrangement was to obtain a great variety of speeds and ranges for the line of light on the strip and so ascertain how far these conditions interfered with the independence of judgment which, a priori, I supposed must exist."
Title: Apparatus for Personal Equation
Credit: Pearson, Karl (1902). On the mathematical theory of errors of judgment, with special reference to the personal equation. Philosophical Transactions of the Royal Society of London, 198, 235-299.
Author: Karl Pearson
Usage Terms: Public domain
License: Public domain
Attribution Required?: No