A Leslie cube is a device used in the measurement or demonstration of the variations in energy radiating from different surfaces. In an experiment described by Tyndall, one of the cube's vertical sides is coated with a layer of gold, another with a layer of silver, a third with a layer of copper, while the fourth is coated with a varnish of isinglass

Isinglass

Isinglass is a substance obtained from the dried swim bladders of fish. It is a form of collagen used mainly for the clarification of wine and beer. It can also be cooked into a paste for specialized gluing purposes....

. When the cube is filled with hot water, the thermal detector (on the far right in the figure), shows much greater emission from the side with varnish than from any of the other three.
See also the description by Poynting and Thomson.

The original Leslie cube was devised in 1804 by John Leslie

John Leslie (physicist)

Sir John Leslie was a Scottish mathematician and physicist best remembered for his research into heat.Leslie gave the first modern account of capillary action in 1802 and froze water using an air-pump in 1810, the first artificial production of ice.In 1804, he experimented with radiant heat using...

(1766–1832), a Scottish mathematician and physicist.