Stanhope Demonstrator

It consisted of two slides coloured red and gray mounted in a square brass frame.

Scales marked zero to ten were used to set the numbers or proportions of objects with the two properties.

[2][3][4] This form of inference anticipated the numerically definite syllogism which Augustus De Morgan laid out in his book, Formal Logic, in 1847.

This opening was called the holon, meaning whole, and represented the full set of objects under consideration.

When the device was used for the "Rule for the Logic of Certainty", the gray slider was inserted from the left.