Hardly any ontological result of modern science is more firmly established than the fact that infrared radiation differs from light only in wavelength; this is part of the modern conception of the continuous spectrum of electromagnetic radiation reaching from radio waves to gamma radiation. Yet, like many such evident truths, the light-infrared unity was an extremely difficult thing to establish. We examine the competing arguments in favour of the unified and pluralistic theories of radiation, as put forward in the first (...) half of the nineteenth century by three of the most important early pioneers of the study of radiation: Herschel, Melloni and Draper. In this part of the paper, we conclude that there were no compelling reasons of observational adequacy to prefer the unified theory to the pluralistic theory. Keywords: Macedonio Melloni; John William Draper; William Herschel; Infrared; Observation; Theory-choice. (shrink)