_Sight_, like hearing, is a telæsthetic sense. Through it we become aware of certain vibratory states of more or less distant objects. The medium by means of which these vibrations are transmitted is not, as in the case of hearing, the air, but the æther which pervades all space. The rate of transmission is about 186,000 miles in a second. That which answers in vision to pitch in hearing is colour. The lowest, or gravest, light-tone to which we are sensitive is deep red, where the number of vibrations per second is about 370 billions (370,000,000,000,000). The highest, or most acute, light-tone is violet, with about 833 billion vibrations in a second. If white light be passed through a prism, the rays are classified according to their vibration-periods, and are spread out in a spectrum, or band of rainbow colours. But different individuals vary, as we shall presently see, in their sensibility to the lowest and the highest vibrations. Some people are, moreover, relatively or absolutely insensible to certain colours, generally either red or green. Such persons are said to be colour-blind. When the rainbow colours are combined in due proportion, or when pairs or sets of them are combined in certain ways, white light is produced. We saw that in the case of sound-waves, when the number of vibrations in a second is doubled, the sound is raised in pitch by an octave. Using this term in an analogous way for colour-tones, we may say the range in average vision is about one octave--that is, from about 400 billion to about 800 billion vibrations in a second. But, though these are the limits in human vision, we know of the existence of many octaves of radiant energy physically in continuity with the light-vibrations. Photography has made us acquainted with ultra-violet vibrations up to about 1600 billions per second--an octave above the violet. And Professor Langley's observations with the bolometer indicate the existence of waves with as low a vibration-period as one billion per second, and even here, in all probability, the limit has not been reached. To the vibrations more rapid than those that are concerned in the sensation of violet, the human organism is apparently in no manner sensitive. But to infra-red vibrations down to about thirty billions per second the nerves of the skin respond through the temperature-sense. We shall have to return to these limits of sensation at the close of this chapter. [Illustration: Fig. 31.--The human eye. Horizontal section, to show general structure.] [Illustration: Fig. 32.--Retina of the eye. Enlarged section of minute fragment. b., back of retina next the outer coat; l.r.c., layer of rods and cones; i.l., intermediate layers; l.g.c., layer of ganglion-cells; l.n.f., layer of nerve-fibres; f., front of retina, the surface turned towards the pupil.]