Pictet`s experiment of 1790 is shown in the top diagram. https://drive.google.com/file/d/1MD1uL1IR0AWBTzxVU2M8vk9bKuKEi4IM/view?usp=sharing
The experiment seemed to show the reality of both hot and cold radiation since both types of radiation could apparently be reflected and focused with parabolic reflectors. The standard explanation is that while hot radiation is real, cold radiation is only apparent because the parabolic reflector nearest the thermometer effectively shades the thermometer from about a third to a half of the warming radiation from the surroundings. The thermometer tends to cool down because much of the heat radiated by the thermometer is absorbed by the cold body as well as receiving significantly less heat from its surroundings. However, the decrease in temperature can also be explained using Rumford`s theory that both hot and cold radiation are real. The bottom diagram is a concept for a new experiment which could meaningfully distinguish between real and apparent cold radiation. This experiment uses one ellipsoidal reflector so that shading from a second reflector could not be used to explain a drop in temperature. The ellipsoidal reflector has the same surface area as one parabolic reflector so the temperature of the thermometer should decrease as much as in the Pictet experiment if cold radiation is real. If the temperature decrease is much less then cold radiation is not real. Harry
