You will notice, on referring to the above table of frequencies, that the range of sensibility of the human eye is very much smaller than that of the ear. Our eyes are wonderful instruments for detecting wave-motion in the æther, and our ears are appliances for detecting wave-motion in the air. The ear, however, is, as explained in a previous chapter, sensitive to air-vibration forming musical tones which lie between 30 and 30,000 per second, and these numbers are in the ratio of 1000 to 1, and cover a range of about ten octaves. The eye, however, is only sensitive to æther-vibrations which lie in frequency between 400 and 700 billions per second, and these numbers are in the ratio of nearly 2 to 1, or comprise only _one_ octave. The question, of course, immediately arises—What are the properties of æther waves the frequency of which lies outside the above limits? Scientific investigation has made us acquainted with a vast range or gamut of æther-vibrations, and we are able to summarize our present knowledge as follows:— The physical effect we call light, and that which we have up to the present moment merely called electric radiation, are really identical in nature, and both consist in waves propagated through the space-filling æther, the only difference between them is in wave-length and wave-amplitude. In between these two classes of radiation comes a third, which is called the dark-heat radiation, and beyond the limits of visible radiation we are acquainted with another group of æther waves which cannot affect the eye as light, but which from their power to affect a photographic plate, is called _actinic radiation_. Hence, briefly speaking, four great groups of æther waves are known to us, called respectively— 1. Actinic, or photographic rays. 2. Luminous, or light rays. 3. Ultra-red, or dark-heat rays. 4. Electric, or Hertz rays. Convincing proof has been afforded that these various rays are essentially the same in nature, and that they consist in periodic disturbances or waves propagated through the æther in every case with the velocity of 186,500 miles, or 1000 million feet, or 30,000 million centimetres per second. We may, therefore, say that these classes of æther waves differ from each other only in the same sense in which a bass note in music differs from a treble one; that is, the difference is a difference in frequency.