As everyone knows, the long waves given off in largest amount from objects at comparatively low temperatures give the sensation of warmth. As we raise the temperature, in addition to these longer heat waves, those of shorter and shorter wave-length are given off in sufficient quantity to be detected. At 525° C., rays of about λ=.76µ in length are just visible as a faint red glow to the eye. As the temperature increases still shorter wave-lengths become apparent, and the light changes to dark red (700°), cherry red (900°), dark yellow (1100°), bright yellow (1200°), white-hot (1300°) and blue-white (1400° and above). Above λ=.4µ the waves again fail to affect our eye, and, although they are very active in producing chemical changes, we have no sense organs for perceiving them. Thus, a white-hot object liberates radiant energy or flux of many different wave-lengths corresponding to what we know as "heat, light and actinic rays." All can be dispersed by prisms of one or another appropriate material to form a wide continuous spectrum, such as that indicated in Fig. 1. Radiant energy of λ=.76µ to λ=.4µ, evaluated according to its capacity to produce the sensation of light, is spoken of as visible radiation or luminous flux. Below the infra-red comes a region of wave-length as yet uninvestigated, and beyond this may be placed the Hertzian electric waves of long wave-length used in wireless telegraphy. Above the ultra-violet comes another region as yet uninvestigated, and then Röntgen rays (X-rays) and radium rays, of exceedingly short wave-length. These last types need not concern us except in that we may later inquire if they are given off by luminous animals. The shortest of the ultra-violet are known as Schumann and Lyman rays. These relations are brought out in Table 2. TABLE 2. _Wave-lengths of Various Kinds of Radiation_ Wave-lengths of light are usually given in Ångstrom units. One micron (µ)=.001 mm.=1000 millimicrons (µµ)=10,000 Ångstrom units (Å) or tenth metres=10^{-10} metres or 10^{-8} centimetres. The entire scale of wave-lengths extends from 10^6 to 10^{-9} centimetres. Hertzian electric waves (upper limit not reached) above 12 km. to .16 cm. Unexplored region .16 cm. to 310µ Infra-red 310µ to .76µ Visible light 7600 Å to 4000 Å Ultra-violet 4000 Å to 320 Å Unexplored region 320 Å to 12 Å X-rays 12 Å to 0.2 Å Radium γ rays 0.2 Å and shorter [Illustration: FIG. 1.--Schematic representation of various types of radiation to form a wide continuous spectrum.]