Those waves in the two octaves higher up, that is, of wave-length less than 0·4μ, are able powerfully to affect a photographic plate, and so, indeed, do some of the waves in the visible octave. We may, in fact, say that all the æther waves with which we are acquainted, the wave-length of which is less than about 0·5μ, are able to make an impression upon a photographic plate. These rays, whatever their wave-lengths, are called the actinic rays. On the other hand, all the æther waves with wave-length greater than about 0·8μ, and for six octaves further down, can only be recognized by their ability to heat a delicate thermopile or other heat-measuring instrument. They cannot affect the eye, and they have little or no effect in decomposing silver salts and impressing a sensitive photographic surface. [Illustration: GAMUT OF ÆTHER WAVES. Wave-lengths in microns. μ = ¹⁄₁₀₀₀ of a millimetre. 0·1μ } Shortest ultra-violet wave-length measured by } Schumann, 1893. 0·2 } Two octaves of ultra-violet waves. 0·4 } .............} 0·43μ = violet light } V I B G Y O R} one octave of light waves. .............} 0·75μ = red light } 0·8 } 1·6 } 3·2 } 6·4 } 12·8 } Seven octaves of dark-heat waves. 25·6 } 51·2 } .............} Longest dark-heat wave-length = 67μ, measured 102·4 } by Nichols in 1898. 204·8 } 409·6 } 819·2 } Five octaves of unknown radiation. 1638·4 } 3276·8 } Shortest Hertz wave made by Lampa = 4 mm. } (1897). .............} 6553·6 } 13 mm. } 26 } 52 } 104 } 208 } Hertz radiation, 12 octaves of most frequently 416 } used Hertz waves. 832 } 1664 } 3328 } 6656 } 13312 } ............. Longer waves still up to 1000 feet or more in wave-length are used in wireless telegraphy. FIG. 80.]