That range of sensibility is, however, very limited. Supposing we allow a ray having a wave-length greater than 0·75 or less than 0·43 to enter the human eye. It produces no sensation of light at all. Accordingly, if we form a spectrum with sunlight, we find a tolerably sharp limit to the visible spectrum. Supposing, however, we allow the spectrum to fall upon a sensitive photographic plate, we find that the plate will be chemically acted upon far beyond the limits of the visible violet end of the spectrum. Hence we learn that beyond the violet there is radiation of a kind which is invisible to the eye, yet can affect a photographic plate. This is called the _ultra-violet_, or _actinic_ radiation. Schumann, in 1893, measured waves in actinic radiation of a wave-length as short as 0·1μ, or one two hundred and fifty thousandth part of an inch, and hence we may say that we are acquainted with at least two octaves of invisible ultra-violet or actinic radiation, or æther waves have wave-lengths lying between the limits 0·1μ and 0·4μ. In a similar manner very delicate heat-detecting instruments or thermometers called bolometers, or thermopiles, show us that beyond the visible-red end of the normal spectrum there is radiation called the _ultra-red_ radiation, or _dark-heat_, which cannot affect the eye. The wave-length of dark-heat radiation has been measured up to a limit of 67μ by Professor Rubens and Professor Nichols in 1897 and 1898. Accordingly, we can assert that beyond the red end of the spectrum we are acquainted with six octaves or more of ultra-red radiation, viz. that lying in wave-length between 0·75μ and 67μ. We may represent the above facts in another way as follows: In most pianos the keyboard extends over a range of seven or eight octaves. Imagine a piano having a keyboard with nine octaves, and that each key was labelled to correspond with a light wave of a particular length. At the extreme treble end let the first key be labelled 0·1, and at the extreme base end let the last key be labelled 51·2. Then the various octaves will be comprised between the keys marked 0·1, 0·2, 0·4, 0·8, 1·6, 3·2, 6·4, 12·8, 25·6, and 51·2 (see Fig. 77). Suppose that each key when struck caused some kind of electric radiator to emit an æther wave whose wave-length reckoned in microns or thousandths of a millimetre, is indicated by the number on the key. Of all this great gamut of æther waves only the notes of one octave, viz. the third from the treble end, the wave-lengths of which lie between 0·4μ and 0·8μ, affect the retina of the human eye as light.