In his monograph "An experimental inquiry into the nature and propagation of heat", published in 1804, the Scottish scientist John Leslie described a series of experiments in which he investigated heat radiation. In his experiments, he used tin cubes of different sizes with side lengths of 3 inches, 4 inches, 6 inches or 10 inches. The cubes were hollow, with an opening in the lid through which they could be filled with water of different temperatures and where a thermometer could be inserted. 

The four sides were prepared differently in the experiments, one side was metallic and polished, a second side was covered with a layer of soot, paper was glued to the third side and the fourth side had different surfaces. The body, now known as Leslie’s cube, was filled with hot water or a cold mixture during the experiments. The radiated heat from each surface was focussed by a concave mirror with a focal length of 0.5 inches (approx. 1.3 cm), which was positioned at a distance of three feet (approx. 91.5 cm) from the cube 

The differential thermometer had a U-shaped capillary with a very uniform inner diameter of 1/50 - 1/60 inch (0.5 - 0.4 mm) on one side and a larger inner diameter on the other. At both ends were equally sized air-filled glass spheres with a diameter of 4/10 - 7/10 inches (approx. 1 cm - 1.8 cm), so that the instrument was closed. The capillaries contained sulphuric acid coloured with carmine. If more heat was radiated onto one glass sphere than the other, the column of liquid in the capillary shifted - the difference in the liquid levels should correspond to the temperature difference in the two spheres. To analyse the amount of heat radiated, the differential thermometer was positioned so that one of the spheres was at the focal point of the concave mirror.

Leslie used this set-up to investigate the radiation behaviour of different surfaces.